ldap/saprdn is a SAP Parameter attribute which is used to control Relative distinguish name for LDAP auto registration 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 value of this parameter contains the relative distinguished name of
the SAP root node up to the system node in the directory. That is, it
specifies the management domain hierarchy in which the SAP system is to
be registered.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.