dbs/db2/ssid is a SAP Parameter attribute which is used to control DB2: subsystem/group attachment 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 .
DB2 for OS/390-specific parameter which allows the DB2 subsystem
name and DB2 group attachment name to be different from the SAP System
name (<(><<)>SAPSID<(>><)>). This value is used to connect to DB2 using
the Call
Attachment Facility and can be either the DB2 subsystem name or the
DB2 goup attachment name unless it exceeds four characters in
length.
The R/3 profile parameter has precedence over the respective
environment variable R3_DB2_SSID.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.