You need to read the reservation section on this site.

Thats it-you there on that funny island in the North Sea-isolationists-always have to pay dearly to use the few trains that can bring you out.

The new batch of hi-speed trains always cost extra-and in some countries now also most of the express trains,as they have mandatory REServations.

But just like In GB- well over 90% of all trains that run in EUR-on those islands and on the continent, are without even the chance to REServe-they are open for you to explore. But also in this forum we see that each&every newbee wants to use those trains and those routes that require extra payment and are also hard to get that RES. So change mindset-or-though you did mot make t very clear if you have boiught a pass or just a normal e-ticket; the pass/app can also set to ´trains without RES´-and then you see what it brings.

In fact this day I made a lovely trip in south Germany along the sunny Bodensee and am now in München-in a Premier HTL (yes, UK brand) with6 different trains-and a replacement bus. without any extra pay

One of the most important things to understand is the difference between reservations and your train ticket (your pass).

Interrail passes are your way to travelling on any train - think of them as a bus pass - it lets you on but doesn’t guarantee a seat. No interrail pass guarantees that. 

Although most trains in Europe are, like most UK trains, simple turn up and go, there are a number that require a mandatory reservation, for which there is often a charge. These are totally separate to your pass and on these trains you must show both your completed Interrail pass (next lesson) and your reservation confirmation. Eurostar is one of these and costs between £30 and £40 pp per journey.

Trains requiring reservations are shown in the Interrail planner with an indication of the fees.

You can also book reservations for many other trains, even if they are not mandatory - example LNER in the UK or ICE trains in Germany.

What do you mean with Eticket? Eurail Pass? Wich is for people not living in Europe the continent (UK is part of Europe) or an Interrail as Mobile Pass?

Second, some trains require a seat reservation, this costs extra money. Most trains are without or only optional reservation in Germany, Switzerland, Austria or Czech Republic. 

Lesson 2 - Using the app:

The app has four main purposes:

1. A train planning tool to check the trains available for your plans,
2. A link to a reservation tool 
3. Your diary of planned journeys - called your trip
4. Your pass manager to create your travel tickets.

Once you have your pass you add it to the app as per instructions in e-mail.

Do not activate step 3 yet.

Create a trip to go with your pass. You only have one trip associated with each pass that holds all your planned journeys for that pass.

As you plan your journeys you add the ones you plan to take to your trip. At this stage consider it like a diary of birthdays - until you put a stamp on the card and post it nothing stops you removing them or adding more.

As you plan you may want to reserve trains and for this you either use the Interrail tool (2 euro per res fee) or use one of the several others available via their websites. These are not shown in the app, although if you buy from Interrail tool they do show a message in your account.

For reasons explained throughout the discussion group the recommendation is not to activate your pass until the first day of your travels.

Finally the key bit - creating your journey tickets.

To create your first travel day you activate the pass with your planned first travel date. That automatically creates your first travel day. You then add trains either automatically from your trip or manually.

This creates a QR code for that travel day, which is basically your ticket. As you travel you add each journey before you board trains to validate the ticket for that specific journey. 

If you change your mind you simply delete that train and add any alternatives. Like a bus pass this isn’t a train or journey specific ticket.

There is no way to get from the UK to mainland UK without reservations. The only train connection is the Eurostar with a limited number of pass holder seats on each departure. The cost for a 2nd class reservation, one way, on the Eurostar is about 30 EUR. 

As there is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance, it is important to make the reservations ASAP.

The best place to see the availability of passholder seats on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations