UMLR — States of Reachable objects may be used in guard constraints, but reachable is not defined

"A guard constraint may involve tests of orthogonal States of the current StateMachine, or explicitly designated States of some reachable object."

Unfortunately, reachable is only used in one other spot in the specification where it is also of no help.

Are all objects reachable? Does it depend on whether there is a chain of relationships, does it depend if the object is in a publicly available package...