For this route your Eurail pass is valid.

You can travel without reservations if you take a regional train from Domodossola to Milan. Otherwise you need a reservation from Brig (Domodossola) to Milan. Reservations on the Swiss part are only optional. 

Thank you very much. I understand that I NEED a reservation for the EuroCity, from Brig to Milan. However, I can’t make the seat reservation for the route on the Eurial.com because the route is not found on the Eurail site.

What can I do?

No, technically you only need a reservation for the EC from Dommodossola to Milan. If you want to avoid that, like @AnnaB said, take a regional train from Domodossola to Milan (IR and R-trains, not EC).

But it’s kinda advisable to get them from Brig of course. You can get the reservations from ÖBB https://shop.oebbtickets.at/en/ticket . Do not select “seat reservation only”, but add to your passenger details the discount “Interrail/Eurail Global pass”.

Via this way you can reserve any train in Italy with mandatory reservations too.

Thank you very much, BrendanDB

I tried ÖBB https://shop.oebbtickets.at/en/ticket 
You mean, add passenger details first, and then “Select seat reservation only”?

When I did, it turned out “tickets not available”. What does that mean? Why can’t I buy the tickets?
So, this time, I tried “One-way tickets and day tickets”, and it turns out € 0,00 for REX. Not EC. and it ways “Ticket for section only”.
I really don’t know what to do.

In addition, I made a reservation for Eurostar and TGV on the Eurail.com site. But now, should I use OBB for Italian trains? I wonder...

Please help me. 
Thank you very much.

Do not select “seat reservation only”. Select "One-way tickets” instead. It's counter-intuitive but that's how the ÖBB site works.

Thank you.
When I Select "One-way tickets”, it turns out € 0,00 for REX. Not EC. and it says, “Ticket for section only”. Thus, I couldn’t make a reservation.
What should I do to make a reservation from Brig to Milano?

Follow these steps:

(Note: Dommodosola should be Milano in my example)

Thank you very much, BrendanDB. How kind you are! It is a great help!
I have a question.

What is the difference between reservations on the eurail site “seat reservations” and OBB site?
Why is making a reservation on OBB recommend rather than Eurail site?
(I’m asking this because I’ve already made reservations on eurail site https://www.eurail.com/en/book-reservations#/ for my other routes, such as London to Paris, and Paris to Switzerland. Did I do wrong?) 

Do I just have two options, Eurail.com and OBB, to make reservation and can I choose whatever is convenient for me? 

From Milano to Venezia, I can still make a reservation on Eurail site as I did for other routes.
Now, I’m confused which one to use and what is the different between the two (Eurail site vs OBB).

Thank you in advance.

 

The price. Eurail always adds a €2 booking fee per person and train and optional reservations are often already much more expensive to begin with. ÖBB don't add any extra fees.

Because it's cheaper.

There are many ways to book reservations, including various websites, by phone, at the ticket office, and you can indeed use what suits you. See this thread:

And this page:

https://www.seat61.com/interrail-and-eurail-reservations.htm

Thank you very much for your help.

From Brig to Milano, the following (the top) is found, two routes of EC.

However, when I click on Journey Preview (in blue), “There are No services available for this Section of the Journey.” like the bottom image. Only the second EC (from domodosola to Milano) can be reserved.

What can I do for this first EC route? As it is EC, I still need to make a reservation, right?

EC trains do not need a seat reservation within Switzerland or up to the border point (Domodossola).

Those 2 trains are actually a single one but ÖBB doesn't show that at the moment (a bug).

Book Domodossola - Milan and sit on your seat from Brig. If it's occupied, either sit somewhere else until Domodossola or gently ask the person to move as you have a booked seat.

Thank you very much for your great help.

1. Do I use the “Rail Planner” app for both the tickets reserved on Eurail and OBB?
2. If I made a reservation on the OBB, what should I show to the ticket personnel? 
3. What is the difference in the way of using tickets reserved on Eurail and OBB? (Actually, I’ve already made some reservations on the Eurail.com site. If I make a reservation for other routes on OBB, I worry that if I’m confused of using those tickets as the reservation site is different.)
4. Actually, I’ve made a reservation from Paris to Strasbourg on Eurail.com site, however, I don’t know how to use it. Can you tell me how? If I make a reservation on OBB, how can I use it?

Thank you very much.

Seat reservations and the pass are completely separate. They'll never appear on Rail Planner, even those made through eurail.com. It doesn't matter where you make them. You only save money by making them on another website than eurail.com.

For all trains : you add the journey on the app (QR-code, your ticket). This can be done at any time, 2 min before boarding is totally fine. You can alter the journey at any point too. The only thing that cannot be cancelled is a pass day as soon as it has started (at midnight).

For trains requiring seat reservations : whenever asked you show both the app (ticket) and the PDF (seat reservation).

Hope it answers your questions :)

Thank you very much for your great help.
1. What do you mean by ‘the app (QR-code, ticket) in the following?
You mean ‘Rail Planner’ app? What do you mean ‘your ticket’? Does it mean my Eurail ticket?

***Do I have to use ‘Rail Planner’ app for both the seats reserved on Eurail and OBB?

2. What do you mean by the ‘app (ticket)’? Do you mean ‘Rail Planner’ app as well? 

3. 

   3. Is the PDF (seat reservation) will be sent to me if I make a reservation?

The Eurail pass  = your ticket. In case of a mobile pass, that is the QR code for each travel day in the rail planner app. Please see:

https://www.eurail.com/en/eurail-passes/eurail-mobile-pass/getting-started

Reservations cannot be done in the rail planner app and won't be visible in it. You'll receive a PDF.

Thank you very much.
The time schedules shown on Eurail.com and OBB are different for my planned route. 
Why are they different? Which one is more correct?
If a schedule appears on any of the sites, that is a possible route?
Thank you in advance.

They have not the same update sometimes. To be 100% sure check the website of the train company that runs the train. 

In your case SBB in Switzerland and Trenitalia in Italy.

Thank you.
I checked SBB site, but some of the time schedules on the SBB are not listed on the Eurail and/or OBB.
But then if a RE time schedule appears on SBB, can I use the route with Eurail pass?
Thank you in advance.

You can take any train of each railway company that’s part of the Eurail/Interrail programme.

SBB and Trenitalia trains, all included. Each of their trains, from the smallest village to the biggest city you can take with the pass.

95% of all trains is included in Europe, some smaller (touristic) railways sometimes not.

See this overview of what’s included and not: https://www.interrail.eu/en/plan-your-trip/tips-and-tricks/trains-europe/railway-companies

Thank you.
Panorama Express of Switzerland is not included, right?
What can I do to use the PE route with Eurail pass?
Thank you in advance.

The Gotthard Panorama Express you mean? Do you have first classes passes or not?

It’s included, but you’ll need a seat reservation (and a class upgrade if you have 2nd class passes).

https://www.interrail.eu/en/plan-your-trip/tips-and-tricks/trains-europe/scenic-trains/gotthard-panorama-express

But you can easily do this route without any extra cost, by just taking ordinary trains. The views are the same.

Thank you very much.

1. What can I do with the following PE route? How can I make a seat reservation?
2. Also, How long would it take for the short transfer in Brig?

Thank you in advance.

This is the Glacier Express. It cannot be booked for such a short journey (or maybe at the prohibitive rate of 49 CHF).

Don't worry, there are 2 hourly trains from Zermatt (no seat reservation needed or even possible) : every hour at xx:13 and xx:37

However if your goal is Milan, no need to take the RE and change in Domodossola (unless you want to of course). Here is the best route :

- RE Zermatt - Visp 07:37 - 08:46

- IR90 Visp - Brig 08:55 - 09:02

OR

- RE Zermatt - Brig Bahnhofplatz 08:13 - 09:37

- enter the main building (easy)

- EC Brig - Milano Centrale 09:44 - 11:40

You book the seat reservation from Domodossola through tickets.oebb.at and that's it. If the journey doesn't appear on the app : either add it in 2-3 steps or manually. No need to do that in advance.

Thank you for the route you recommended. I’d take it. 

1. Is the 7 minnute transfer time enough from Brig Bahnhofplatz to Brig? (09:37-09:44 as shown below) How far is it? Can you show me the way for transfer?

2. What do you mean by “No need to do that in advance”? Do you mean that I can make the reservation even on the travel date or a day before? 

Thank you very much.

.