rdisp/msserv is a SAP Parameter attribute which is used to control Message Server service 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 .
Name of the service where the message server can be reached. A default
name is created from the SAP system name. This entry must be the same on
all application servers belonging to one SAP system and for this reason
it is to be set only in the default profile. The service must be defined
in the services database.
UNIX
Must be entered in the /etc/services file
Microsoft Windows
Must be entered in the file C:\WINNT\SYSTEM32\DRIVERS\ETC\SERVICES
See SAP Parameter Documentation for full SAP documentation for this profile parameter.