Formatted Search to imput TaxGroup Automatically when create a new item

I create a FMS to imput the taxgroup when create a new item. But when execute the FMS this show a list of values. Whats a problem?

declare @iva5 VARCHAR(10)

declare @iva12 VARCHAR(10)

declare @iva21 VARCHAR(10)

set @iva5='D1'

set @iva12='D2'

set @iva21='D3'

SELECT CASE

WHEN T0.ITMSGRPCOD IN ('107','108','109','110','111','116') THEN @iva21

WHEN T0.ITMSGRPCOD IN ('112','113','114','115') THEN @iva12

WHEN T0.ITMSGRPCOD IN ('105','106') THEN @iva12

ELSE @iva12 END

FROM OITM T0