The Source Set deletion is part of running a reference pattern where the set of sources for the reference is deleted.
More on running reference patterns can be seen here: 8.0.x: CMS - Dependencies - Rules tab
The loglines generated by this task is as follows
CAST-MS log file
Section of CAST-MS log file dealing with Create Source Set step as shown in the log above.
Starting Delete Source Set
INF: 2016-09-13 16:35:09: starting Task Delete source 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 source set for My Reference Pattern