Where to enforce requirements

Requirement: the name variable in class X has a maximum length of 10 characters.

Should class X even know about this requirement, or should the requirement be enforced solely by the business logic class?

Well, business refers to one of the 3 layers of design separation...

http://en.wikipedia.org/wiki/Business_logic

What do you call it? It's just sort of a way to make talking about abstract concepts easier—a bit like design pattern names.