You have to match your travel with the trains you put in the pass, if plans change or you are delayed you can delete a train and add a different one.

You can do this as much as you want, whenever you want but it is strongly advised to make sure you add a train before boarding or immediately on boarding if you haven’t been able to do beforehand.

That is fine thank you! Just wasn’t sure you could do it only a few minutes before the new train, but if you can then brilliant! Thanks!

The reason behind this is that in some countries (GB!) and on some lines there is more as 1 company running and in the end this report was meant for the statistics to divide the intakes re what lines people use. But this is really enormous fine-tuning. In actual practice very often there will simply be no check at all, but in some countries they seem to be much more easily distorted about this as in others.

The best way to check if you need a reservation is by using our online timetable. Once you’ve entered your journey details and the results are shown, you may see the note "SEAT RESERVATIONS REQUIRED". If you see this, you must reserve a seat or a bed for that train. This is more difficult to check for trains in Great Britain as they do not share all their timetable data with us. You can check out the full timetable on National Rail and add the journey manually if it's not available in the app. Reservations can be made locally at the station or via ACP Rail. 

@Claudi. 

Unfortunately, the online Eurail/Interrail timetable is only slightly less unreliable than the Interrail app, including incorrect information about required reservations. E.g. the direct EC trains Prague - Budapest, where no reservations are required.

In GB, seat reservations are not required in any day trains. Further, it's not a good idea to use ACP Rail for day train seat reservations in GB for €6 per seat, since they can all be done free of charge via the GWR website.

Therefore, please unmark your comment as the best answer.