The Presort BU has a reference which is required (Ref: Site Code) and users often mistake to put it on the EL, and they just add it to SO at the time of building loads.
Note that in Presort, teh build loads from scratch more often that FDR.
Please copy the same rule set we have for copying PO from SO to EL, for Ref: Site Code
Geno applied the exact same DRS we had for PO Number to Site code as well (both in QA and Prod)