Re: Constraint on Category > why via Referenced Item Type?
The purpose of the constraint is to restrict the category value based on the type of content the node (requirement) is pointing to (the shared item). The shared item could for example be a Document or a Shared Requirement and usually the constraint you identified below is always paired with a similar document version of the constraint.
Source Field: Referenced Item Type
Source Values: Requirement Document
Constrained Field: Category
Allowed Values: Document
So to answer your question: A node is always going to be the same type, but the type of the shared item can vary.