rspo/spool_id/max_number is a SAP Parameter attribute which is used to control Upper limit of spool ID number range 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 .
This parameter sets the maximum possible spool number in the system. The
value cannot be changed by the customer. The only value allowed is
99000, the same as rspo/spool_id/loopbreak.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.