If the item is made by the Players. Item has "Optimal Use" IE You would be best to be rank 40 to equip an item and if your under that you get diminishing returns.
If item is dropped by a NM or Quest the item is Restricted to rank 40 and you can't use it unless you are 40 or over.
Because the game is focused around crafted gear a majority of it would be Optimal Use, but the special items like AF's, Relics, RSE's, Etc would all be Rank, Class or Job locked.
Thats how it should be.