regd autherisation code for a field

Hi All,

i have one user-exit requirement in VA01 . in va01 i need to provide the autherisation on ORDER-REASION field.

ex:in order resion field F4 (have 5 option there, autherised user need to secect one then document saved else it will the error.

based on my autherisaion code basis will create a role and they will assign the users.

is there any special code to provide autherisations for users.