IBM 5170 Technical Reference page 398

Startup
At startup, a call is issued to determine if the SYS REQ key
support is already loaded and to initialize the support for the new
subsystem.
The parameters for the startup routine are the address of the
entry point and the function code (direct-access mode). If the
operation was successful, the carry flag is set.
The following shows the calling sequence.
MOV AX,SEG entryJoint ;address for SYS to call
MOVES,AX
MOV BX,OFFSET entryJoint
MOVCX,XXXX ;super shift mode code
MOV AX,2010H
;AH=20H, AL=10
INT 1SH
If the carry flag is not set, the initialization code needs to hook
the vector for interrupt iSH, save the previous address, and
reissue the initialization call.
Activation
This is a signal from the SYS REQ processing module that a
subsystem's monitor is to be activated.
This entry into the subsystem dispatcher signals that the monitor
task should be activated. It should be treated as a signal to set a
flag for the subsystem rather than an opportunity to gain control
of the system asynchronously as it may not be a proper time for
the subsystem to run. The subsystem may have to wait until a
higher priority subsystem allows it to have control before the
subsystem's monitor gets control. The subsystem entry point is
CALLED with the AH register set to O.
9-20 Compatibility