rdisp/start_icman is a SAP Parameter attribute which is used to control start internet communication manager 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 .
For an application server to directly process external processes such as
HTTP, the Internet Communication Manager (ICM) must be started. The
parameter rdisp/start_icman is used to control the ICM startup:
rdisp/start_icman = true ICM is started
rdisp/start_icman = false ICM is not started
See SAP Parameter Documentation for full SAP documentation for this profile parameter.