If the response being analyzed is the only response form typically evoked by a particular variable, then we do not call that variable a main thematic source. A main thematic source must strengthen two or more responses and all of the multiple response forms must have essentially the same effect upon the listener.
A situation with the following features:
It controls a large group of responses
All of the responses would have about the same effect on a listener
Type of thematic elementary relationship (e.g., mand, tact, intraverbal)
Specific features of the stimulus and response