All the symbolic commands used to control I/O can be found in:-
Companion/Coding/The SNOMAN Symbol Table/Job control
As from 2.05, SNOMAN uses Zebra's FZ Exchange Format facilities which means that data written on one platform can be read directly on another. The user must inform the SNOMAN whether the device is tape or disk, by means of the DEVICE FILE option, see 3.9.5. By default SNOMAN assumes disk. A third, device type, mem, was introduced in 3.02. This is used to connect SNOMAN to an event stream dispatcher, and is described briefly in section 21.3.