The Target Set deletion is part of running a reference pattern where the set of targets for the reference is deleted

More on running reference patterns can be seen here:

8.0.x:  CMS - Dependencies - Rules tab 

CAST-MS log file

Section of CAST-MS log file dealing with Create Source Set step is shown in the log below. 

Starting Delete Target Set 

INF: 2016-09-13 16:35:09:              starting Task Delete  target set for My Reference Pattern

INF: 2016-09-13 16:35:09:              Task message: No Task message

INF: 2016-09-13 16:35:09:              No associated log file

INF: 2016-09-13 16:35:09:              ending Task Delete  target set for My Reference Pattern