ApplyMap('SFIDmapping_SF',[Level 3 Category],null()) as [SF ID]
The above field SF_ID was created by ApplyMap function with SFIDMaoing_SF mapping table.
First read about Mapping tables and ApplyMap function.
Generally Mapping table have 2 fields like Key & Value pair, When you used ApplyMap, the key found its assign the Value to new field in the script.
You can check Level3 Category field exactly match with Mapping table First field values to debug why you got null values.
Also, these are two great posts on ApplyMap.
Henric Cronström: Qlik Design Blog : Don't join - use Applymap in... | Qlik Community