When an OPEN or USE command includes no mnemonic space argument, Caché uses the default mnemonic for that device type. The Mnemonic_MT setting specifies the default mnemonic for magnetic tape.
The value n is a Caché routine name. ^%XMAG is the default for magnetic tape. You can use this default, or reset it by changing the value of as needed. See the “Controlling Devices with Mnemonic Spaces” chapter in the Caché I/O Device Guide.
As described above.
On the page System Administration > Configuration > Device Settings > IO Settings, in the in the MagTape row, enter a Caché routine name.