The Source Set creation is part of running a reference pattern where the set of sources for the reference is created.
More on running reference patterns can be seen here:
8.0.x: CMS - Dependencies - Rules tab
Following is the set of loglines related to this task.
CAST-MS log file
Section of CAST-MS log file dealing with Create Source Set step as shown in the log above.
Starting Create Source Set
INF: 2016-09-13 16:34:53: starting Task Create source set for My Reference Pattern
INF: 2016-09-13 16:34:54: Task message: Number of objects selected : 8867
INF: 2016-09-13 16:34:54: No associated log file
INF: 2016-09-13 16:34:54: ending Task Create source set for My Reference Pattern