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