SRCFILE is used to specify the name of the external source file. See Section 3.2 for the required formats. The default SRCFILE is $SCHED/catalogs/sources.vlba. Assuming the environment variable $SCHED is set (see below), this is the usual SCHED source file. If SRCFILE='none' is specified then no external catalog will be read. The file name is simply passed to the operating system so all the usual behavior you expect normally apply, such as the use of the current directory if a path is not given.
Under unix, environment variables may be used. For example, if SCHED is defined to mean /users/cwalker/sched, the base area under which all SCHED stuff is kept (substitute your local directory), then one can specify the source file with SRCFILE = $SCHED/catalogs/sources.vlba (which happens to be the default). Use the setenv (c or t shell) or export (korn shell) to set environment variables.
There is an option to use a second source catalog file using SRCFILE2. This could be useful if one wishes to get calibrator information from the SCHED standard catalog, but also have a catalog of program sources. It is also useful when using the ability to specify many phase centers per 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.