rsdb/db2jcl_library is a SAP Parameter attribute which is used to control DB2: Full path name of library for JCL submission service 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 specifies the library for the JCL submission service. This profile parameter has precedence over the corresponding environment variable RSDB_DB2JCLLIBRARY, and one of both is required.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.