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: 8.3.x: CMS - Dependencies - Rules tab
The loglines generated by this task is as follows
Excerpt Macro - Logs - Full Analysis Image for 83
CAST-MS-<date>-<time>.log.txt 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: 2018-11-05 16:47:28: starting Task Delete source set for ref1
INF: 2018-11-05 16:47:28: Task message: No Task message
INF: 2018-11-05 16:47:28: No associated log file
INF: 2018-11-05 16:47:28: ending Task Delete source set for ref1