First Class Eurail Pass and Elige Confort Seat Reservation

Hello!  We have purchased 1st Class Eurail Passes for Spain.  My understanding is that this allows you to sit in the first class car of the train.  I’m not sure if there is any other benefit???   I have purchased first class seat reservations, when required, for each train that we’ll be taking.   The seat reservation ticket says Elige Confort.  So, a couple of questions… 1) How will I know what cars of the train are First Class?  2) Are there seat assignments in the First Class car or are we able to sit in any seat of that car?

Thank you for any info you can provide.