SRCFILE2 is a second source file, treated identically to SRCFILE. This allows one to supplement the main SCHED catalog with a local, but still not embedded, catalog. It is likely to be especially useful for observations wanting multiple phase centers in each pointing center.
SCHED will use the data for a source from the first catalog it reads that includes that source. It reads the in-line catalog first, then SRCFILE and then SRCFILE2.