On the TER Strasbourg-Basel you can’t make any seat reservations. Within Switzerland you can, but I generally haven’t had much issue with reservations - to be fair I’ve usually travelled there during Summer holidays, not sure how it is now. Generally, unless you’re with a few passengers and want to sit together, you should be able to just get on board and find a free seat. Plus, as the TER’s aren’t extremely reliable it’s possible you’ll miss your reserved itinerary anyway.

Hardly anyone makes reservations on these trains so you just take any free seat. If seats are reserved there should be a sign with information about the reservation above the seat.

This system of labelled seats applies to about anywhere in EUR when RES is not mandatory-and yes, I understand this is far from practice like it is on VIA or ONR in Canuckyland. And also yes-well over 90% of all trains running in EUR do not even have any chance to RES just like f.e. the GOtrains around Tronno or those into Vancouver. As a rough rule less as 50% of seats offered are occupied-but this varies enormously per line, time of day time of yr etc. These eternal fear about full trains are overhyped nonsense-except for a very few that about any new passuser seems to want to use.