Hi Andrew,
as far as I know, this command is only for purging unprotected media from B2D (StoreOnce Software and maybe Catalyst-connected store) devices, not for anything else. I wasn't aware it's even available in 6.1. Good old file libraries would purge expired media on write attempt anyway, so this command was specifically introduced for B2D devices, which aren't. It can be run manually, but also runs scheduled (per default, at high noon).
Just call it with the name of the B2D "library" device that is to be purged, or -all, in which case it goes about purging all configured B2D devices at once.
HTH,
Andre.