SAP CRM Service Request subject value determination
When a new Service Request is created, the multi-level drop down list for Service Request subject is available.

Where do the value for those drop down list come from?
Here below is what I found via debugging:
get allowed category type from table CRMC_PROC_CATTYP based on transaction type:
Two entries found:
These two entries are inserted here:
Then check for the given combination of Transaction type plus Catalog category, whether there is existing schema defined.
The check is done by API CL_CRM_ML_CATEGORY_UTIL=>GET_ASPECT.
I wrote a small report to check whether a schema is available for corresponding category:
In my example, no schema exists for category C:
But a schema exists for type D:
the ASP ID YBP_SERVICE comes from here: