When adding a gift in a donor's record, the Primary Constituent Code is the default Constituency listed on the gift.

Displaying the constituency code in the header for the record in Constituent Management uses the primary code.

In Export, if you export out constituent code, you might export out 3-4 codes in order to get the one you want. Where if it is the first for everyone then when you export you can leave the "number of constituency codes to export" as 1.