dbs/mss/schema is a SAP Parameter attribute which is used to control Schema name (or sql server user name). Always SID in lowercase. 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 schema systems (MCOD), this parameter is used to specify the schema name which must always be equal to the SID in lowercase.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.