Ordinarily you can reserve a seat & you can even choose a specific carriage / seat while making a the reservation. If you do a dummy booking for a date prior to the proposed new timetable (about 14 Dec 2019) you'll see how it works.

For dates beyond the not yet finalised timetable change you can make a booking but not reserve a seat. I thought this would get you a place on a specified train but from your experience it looks like you just get an open ticket for any train.It seems to me that rather than do that they shouldn't be taking bookings, what's the point? I guess they're doing it as some trains might not run when the new timetable is finalised.

Personally, I'm checking every day and hopefully the timetable will be set soon so I can reserve a specific seat on a specific train. I'll post here if/when I have success.