icm/listen_queue_len is a SAP Parameter attribute which is used to control Maximum number of requests in state waiting for connection information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
The operating system must keep requests in a queue while the connection
is being set up. This parameter specifies the maximum number of requests
than can have this status at any one time. If the network queue is full,
the system rejects any additional connection requests without response.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.