Given the definition that a relation on AXA is a subset of AXA, what does "R is a relation on a set A" mean?

a) R is a subset of AXA every member of which satisfies R (aRb).

or

b) R is a subset of AXA consisting of all (a,b) belonging to AXA st aRb.

Note both statements satisfy the definition that R is a subset of AXA.