You can add new journeys as you go along, also on the same day. You can also delete existing journeys, or separate trains from a journey. You don't have to plan in advance if you don't want.

EXCEPT when the new train has mandatory REServation-in such a case you may be lucky or not to find empty space. But it seems from this forum that about any newbee wants to use exactly those trains and then gets stuck. But f.e. in DE=Germany, you landed in hamburg and then you think: no, not Berlin today, Iĺl go to Dortmund: thats quite OK. Or not the direct train to B, but first to Hannover-have a look, then on to B: also OK.

Mobile pass has same options as old-style paper pass (on which I am right now, here in Praha), you can write just the trains you take right before-or right after sitting in.