The Cuff is the upper part of a skate that flexes forward. There are no cuff standards unfortunately so skaters are mostly stuck with the aftermarket cuffs that their own brand offers with these exceptions:

• Rollerblade and Roces have a shared history in the 80's and still have the same cuff 
• FR and Seba have split up in 2018 and also have the same cuff system for their FR model and CJ/SX model skates. These cuffs do have sizes, but for the 2-4 and 5-8 cuff this only increases the height of the cuff, and does not change whether they fit or not.
• Razors and Remz have a co-ownership and share the same Cuff system ever since the Remz 2003 models came out.