how to write accounting rules using SOURCE_TYPE_CODE = 'EXTERNAL' on purchase requisitions

Summary

Need to write custom accounting rules where condition = SOURCE_TYPE_CODE = 'EXTERNAL'. For 'INTERNAL', it should fire standard rules.

Content

Need to write custom accounting rules where condition = SOURCE_TYPE_CODE = 'EXTERNAL'. For 'INTERNAL', it should fire standard rules.

Cannot find a source in purchasing for SOURCE_TYPE_CODE

Kindly help.