Any day you board a train uses a travel day. It does not matter if it's a domestic or an international train.

Note that you have to also account for around 80€ in extra to pay supplmt/RES-fees. Which makes me wonder if a pass would have been cost effective-but its bought. Do that RES for €* asap-many post about not being able to find seats with pass on it.

YES, OUT of home country you can do whatever you want-on those trains that accept passes-for a very few others (NOT in your trip) you have to pay the normal fare. And yes, there are also smart people from out of that island who use such a pass to beat the sky-high BritRail fares. 

I found a way to be flexible and buy a cheap full fare ticket (example paris - lyon) that still cost less than the the RES fee using the pass. therefore I can save that travel day for more exploration!