icm/host_name_full is a SAP Parameter attribute which is used to control Full qualified host name 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 .
You use this parameter to specify the fully qualified host name (FQHN)
of the host on which the ICM is running, and which can be contacted for
requests. For example, saphost5.sap-ag.de instead of saphost5.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.