DD_UPDATE, DD_INSERT, ... are only readable names for constants used by Update Strategy :
So the Update Strategy simply uses the return value of the expression, whatever is returned.
For this example, suppose different values of $$export_agreement :
•If $$export_agreement is 0, the return value is 1, which means Update.
•If $$export_agreement is 1, the return value is 1, which means Update.
•If $$export_agreement is 2, the return value is 0, which means Insert.
I don't know if it's what was intended, but I would suggest you write it in a more explicit way :
iif( $$export_agreement =0 OR $$export_agreement =1, DD_UPDATE, DD_INSERT)