rsdb/MB_CUR_MAX is a SAP Parameter attribute which is used to control Maximum bytes per character of the DBMS characterset 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 .
Explicitly specifies the MB_CUR_MAX (maximum bytes per character) of the DBMS when overridden with value 2. Unless in exceptional cases where the DB-interface fails to recognize the DBMS characterset as being multi-byte, the parameter should remain at its default value, in which case the DB-interface attempts to detect the correct value for MB_CUR_MAX of the DBMS characterset. Value 1 yields to automatic detection of the correct value, whereas value 2 forces the DB-interface to recognize the DBMS characterset as being multi-byte. This parameter affects handling of underscores ('_', 0x5F) in WHERE clauses.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.