Specifies the subchannel set in which the command is to be issued.
The subchannel set where the I/O will be issued is the subchannel
set as defined in the Hardware Configuration Dialog (HCD). The valid
values are determined by what is supported by the host system's processor
and configured for the device. If you omit the parameter, the command
is issued to the device that is logically in subchannel set 0. If
a swap has occurred, this could be the device that was defined in
an alternate subchannel set in the I/O configuration (IODF), with
HCD.