UMLR — semantics of associating a use case with another use case, or indeed anything other than an actor, are unclear

The semantics of associating a use case with another use case, or indeed anything other than an actor, are unclear. There is a rule specifying that use cases may be only be associated with other use cases with different subjects because they describe a complete usage of the subject. But that doesn't explain what it means to have any association. The only hint is in the notation section that gives some examples as "(e.g., to denote input/output, events, and behaviors)." However these details ought to be part of semantics and expanded on.