Thank you for your reply. I think I may understand. Let me give you an example.
Say I want to book a ticket from Tirano to St. Margrethens.SG using the SBB web portal I can see two services:
1 - Depart 10:06 with changes shown at St. Moritz and Landquart, and
2 - Depart 11:00 with changes shown at Pontresina, Samedan, Chur and Sargans
When I click on “Select Tickets” I am taken to the next screen this shows the Start > End and “via” - which is the same as the number of changes.
This is then shown on the ticked and you are therefore restricted to any train of that day on that precise route - is that correct? This will apply if you are paying full or half price, but of course if you have a Swiss Pass then you can take any train.