There is no one answer, it depends on which country and operator.

This is a good place to start:

The rail planner app cannot book any reservations. It only forwards to some of the websites where you can book.

1. Let's say that I'm in Switzerland trying to book a seat reservation, which app should I use? Reservation tool on interrail’s website, SBB or any other carrier like OBB or DB?

2. The website has a reservation tool I think

Thanks for the answer btw!!

In Swiss trains, almost nobody books reservations. IMO, reservations in Switzerland are a waste of money.

For other countries or operators, please see the thread that @Al_G mentioned. If you still have questions after looking into that, then please mention route, date an departure time of any trains you'd like to book.

The Interrail website does indeed. But it often has problems of various kinds, it charges extra fees, and sometimes can only issue paper reservations when other websites can issue electronic reservations. It's generally the last choice to book.

If you travel in Switzerland with domestic trains, you don't have to reserve. It's not even possible (with some exceptions for touristic trains, like the Glacier Express and international trains).

I always like to reserve at the operators website of the specific train you're taking. So reserving via DB if you're taking ICE, on ÖBB if you take Railjets. You usually get updates on real time information, via 3rd party websites never.

If cost is an issue, that ÖBB and CD can be cheaper than interrail or DB for example. All information in the reservation topic linked above! 

On long distance trains in Switzerland like IC or IR, reservations are possible, but only tourists that do not know that it is optional do a reservation. When you are travelling as a big group it could be a good idea. 

Seat Reservation App helps employees to make reservations for work station and cafeteria seats. The app is designed to manage the reservations on the go.

With the Seat Reservation App employees can:

• Workstation and cafeteria seats - Create, view existing reservations or cancel it

• See real-time availability of seats

• GUI based floor plans for office space make the reservation much easier

• Attendance marking using QR code

• Organizational announcements

Which app you talking about? 

Don't bother about the reply above. I have reported it as spam. 

Bonjour, 

Pour notre voyage de 3 semaines en Octobre, nous avons utilisé principalement OBB, site tres clair, complet et rapide, ainsi que trenitalia pour l Italie. 

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