Communication.instantiates is of type uri but refers to the protocol or definition it instantiates. For example, a PlanDefinition.

PlanDefinition has a canonical URL (.url + .version), so Communcation.instantiates should be of type canonical to be able to reference the correct business version of the protocol or definition that was used.

And hence the related searchparameter 'instantiates' should be of type Reference.

This may also be applicable to other resourcetypes following comparable workflow patterns.