abap/string_sharing is a SAP Parameter attribute which is used to control Switch on/off ABAP SHM String Sharing 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 .
Enables you to switch string sharing between shared objects areas and
between the shared objects area and the roll area on and off.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.