alert/MONI_SEGM_SIZE is a SAP Parameter attribute which is used to control Number of bytes for CCMS monitoring infrastructure (in SHM key 73). 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 .
Parameter sets the size of CCMS Monitoring segment. If the parameter
value is set to 0, the CCMS monitoring infrastructure is disabled.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.