One main difference with 3H and older games is in the promotion paths. In 3H, you have the ability to make any unit whatever class you want. Sure, you’d probably rather have Lysithea as a Gremory than a Fortress Knight, but the option is there if you want.

In most of the older titles, your units had very set promotion paths. If you were an Archer, you were going to promote into a Sniper. If you’re a Myrmidon, you’re going to promote into a Swordmaster. There was no reclassing. In most classic games, you have two tiers of classes: unpromoted classes and promoted classes. Your unpromoted class has a level cap of 20, but if you use a promotion seal (which works when you’re Level 10 or above), then you’ll promote to Level 1 in your promoted class, and that class also has a level cap of 20.

I’m not sure what you found confusing as it’s very straightforward, especially since everyone has a set path.