On the Atari QL-Emulator, this command switches to Atari mode. Naturally, on other machines, it has no effect. It will also fail if a QL ROM was found at the start address of the ROM-TOS ($FC0000) - it is possible to load QDOS to that address.
The FN Toolkit (pre v1.04) contained a function of the same name which had a different effect - this has now been renamed QuATARI (see below).
This command will most probably fail on the latest ST/QL drivers.