What does the percent sign indicate in abap function parameter?

Question

I have encounters this abap snippet that I have difficulty to comprehend.

call function 'com_invoke'
   destination pv_wrkdest
   exporting
        %instid         = pv_instance
        %method         = 'FieldCountGet'
   importing
        %return         = lc_return
        fieldcount      = pv_fieldcount
   exceptions
        communication_failure = 1
        system_failure        = 2
        invalid_instance_id   = 3
        others                = 4.

What does the percent sign indicates in the %instid, %method and %return parameters?

Answer 1

Could it just be an (obsolete) naming convention? I.E. are all the parameters that start with % either "Pass by Value" or "Pass by Reference"? I've just tried to create a parameter like that in a test FM and SAP won't allow it.