How do I Find what populates the custom field vendor.shortchar01

Our EFT script is not working properly as it does not populate the vendor.Accountref field. It has the code below . Can anyone tell me how to find out where the custom field vendor.ShortChar01 gets populated. Its not on the supplier customer maintenance form.
vendorAccountRef = if Vendor.ShortChar01 <> “”
then caps(trim(substring(REPLACE(Vendor.ShortChar01, “,” , " "),1,12)))
else caps(trim(substring(REPLACE(Vendor.AccountRef, “,” , " "),1,12)))