rsau/selection_slots is a SAP Parameter attribute which is used to control Number of selection slots for security audit 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 .
The parameter specifies the number of selection units that are
set using Transaction SM19 and checked by the system during
processing.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.