Customizing Order Booking Form

[markuh]

:LINE.ORDERED_ITEM_DSP IS NOT NULL
AND
0 = (select COUNT(ol.ordered_item)
from oe_order_lines_all ol, XX_ITEM_CARE_LEVEL_V xx
where ol.header_id = :LINE.HEADER_ID
and ol.line_id = :LINE.LINE_ID
and xx.ordered_item = ol.ordered_item
and xx.party_site_name = :LINE.ORDERED_ITEM_DSP
and xx.party_site_name = :LINE.SALESREP)
and :LINE.LINE_CATEGORY_CODE <> 'RETURN'
and :LINE.FLOW_STATUS_CODE = 'ENTERED'

is the code i have used to stop orders to be booked for certain customer levels.
i.e. in my view XX items are assinged to customers and therefore if the item has no record in XX, the order must be cancelled and user ginev that message.
how should i make this work, when user wants to move to Book order stage