rsau/max_diskspace/per_day is a SAP Parameter attribute which is used to control Maximum size of all security audit files per day 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 maximum size of one or all Security Audit
files each day. In order that an absolute maximum can be specified that
cannot be exceeded.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.