Position Restriction

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 how to restrict the positions in assignment screen; ex:  If i select one BU, Department, now if i go to position field will this restrict that only positions available under this Department should display.

Ex: I have 2 Positions

Position : 1) Position Name: Specialist  , Position Code: 205,   -  Mapped to BU is A, Department is ENT

                2) Position Name: Specialist  , Position Code: 412,    -  Mapped to BU is A, Department is Gynic

Now In Assignment screen for one employee; first select BU is A, Department as ENT, Now if i go to position field;  system is allowing to display both the positions code of 205, 412,  how to restrict ,  can I able to see only one position which is mapped to ENT department which is of only 205 position code. 412 position code should not be available in the list at all.