Relation between BAPI and transaction

Hi,

In transaction VA42 (Sales contract change) there is an option called CREATE WITH REFERENCE..It's a button.

Does anyone know how this button is linked to the BAPI

bapi_contract_createfromdata or bapi_contract_changefromdata. Maybe there will be a field which has to be filled with an 'X'. I couldn't find it. Does anyone have any idea about this