Definition - What does SPAU mean?
SPAU is the transaction used for adjusting repository objects (if necessary) during the upgrade process or during the import of support packages. This activity is usually completed after the processes involving adjustment for database objects by transaction SPDD and Unicode conversion. The activities involved in SPAU are considered critical during an upgrade, which may be either technical or functional. The scope of objects involved using SPAU includes program, screens, function modules, classes and other enhancement objects. The phase involving SPAU transaction usually comes in the downtime phase of the SAP upgrade and can be carried out in the normal client. This is unlike other upgrade processes like SPDD and Unicode conversions.
Techopedia explains SPAU
During an SAP upgrade or in the import of a support package there is the possibility that the existing SAP objects will be overwritten. In cases where users want to retain the modified objects, an SPAU transaction can be used. All modified objects are provided in SPAU and can be adjusted there as well. Any change to the object, either through manual adjustment or through SAP OSS notes, is identified as a modified object and would be provided in SPAU. Adjustments to the objects are done through consultation with technical and functional SAP resources and are done through a manual process in transaction SPAU. For all objects, the option is to either maintain the previous changes through "Adopt changes" or to recommended SAP changes through "Reset to Original." After the changes are made to the objects, they can be applied to other SAP systems if needed through transport requests, which are generated in SPAU.