User Exits for Devt. Class MEW - BAPIs
If you create a reservation with the BAPI MaterialReservation.CreateFromData (function module BAPI_RESERVATION_CREATE), you can use this enhancement to check all items and change them, if necessary, prior to saving.
All field checks then take place after this function exit is called. If
EXIT_SAPLMEWB_001: Customer Exit: Creation of Reservation via BAPI_RESERVATION_CREATE