Are you familiar with the basic setup for a conditional value list?
If not, please check out this knowledgebase article on conditional value lists: http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list
It looks like you'll need two different relationships to different table occurrences of "ABC". One would relate by part number and the other by product description. You'd use the description field based relationship to control the conditional value list and the part number based value list to look up ( copy ) any values from ABC that need to be copied into your table.
If "Table Occurrence" is a new term for you, see this thread: Tutorial: What are Table Occurrences?