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: CMS - Dependencies - using Reference Patterns
The log lines 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 Delete Source Set step is as shown in the log above.
Starting Delete Source Set
INF: 2018-10-23 16:42:09: starting Task Delete source set for ref1
INF: 2018-10-23 16:42:09: Task message: No Task message
INF: 2018-10-23 16:42:09: No associated log file
INF: 2018-10-23 16:42:09: ending Task Delete source set for ref1