abap/extrsyn is a SAP Parameter attribute which is used to control External RSYN (for interal use only) 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 .
Name of the file containing the formal description of the ABAP language.
Use: Configuration.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.