DOPSRC is the name of the source for which the Doppler calculation should be done. This is allowed to be different from the source being observed to allow for observing continuum calibrators at the same frequency as the line source. If it is blank, the source being observed will be used. If it is not specified, the previous value will be used so it must be set to blank after use in earlier scans if it is desired to return to the original default.
A warning will be issued if SOURCE and DOPSRC are different and both have velocities specified (assumed to be line sources). This is a valid but unlikely observing style so it is not blocked. But too often it has been the result of ignoring the warning above about the defaulting behavior of DOPSRC.