When working with tape data sets, there are two alternative ways
to specify the device unit on which the tape is to be mounted.
Normally this can be done through the use of the site
unit, but in some situations it is not practical to do so.
The environment variable
SSH_SFTP_VOL_UNIT_MAP can be defined to
provide a mapping between a volume serial number pattern and a
device unit, allowing the unit to be deduced from the volume.
SSH_SFTP_VOL_UNIT_MAP may be set to a picture-clause type pattern
matching a six character volume serial number, and followed by the unit to be used, in the form
A is an alphabetic character
9 is a decimal digit, means that volumes with a serial
number consisting of two letters followed by four digits are to be
an allocation request for a tape with volume serial number