Hopefully my poor attempt at solving this does not confuse things more
1. I am using a select to return a value 'CA_QTY'
2. If input1 (UOM) = 'BX', then divide input2 (QTY) by CA_QTY
3 If input1 (UOM) <> 'BX' then output input2 (Qty)
WHEN :UOM = 'BX'
(Select :QTY / CA_QTY)
(select Item_qty_Bkd_qty CA_QTY
from M_item_D1 where comp_code = 'A1' and cust_code = 'CLIFBAR'
and item_code = '120004'
and ITEM_QTY_BKD_LEV_NUM = 2)