Notes

You do not provide this environment variable. BIS sets the environment variable when a service program is started and creates the file when B$ReadRequest is called.

A separate file is created for each service program, and the same file is used by

B$ReadRequest to receive requests from BIS.
B$WriteResponse to transmit responses to BIS.

While the filename is already known when the service engine is started, the file itself is not created until B$ReadRequest is called for the first time by the service program.