Plant is optional in NBF Field selection key, then why I am getting a hard error messge "Enter plant"?

Hi Gurus

NBF is the field selection key corresponding to "NB" doc type of Purchase Orders. But in "define screen layout at document level", I am seeing the field "plant" optional, NOT required. Then why I am getting a hard error message "Enter plant" while creating a NB plant order. Is not the field "plant" showing to be mandatory ?

Thanks,

Kumar