Whenever I write that in my "Issued for Bid" set with regards to the lighting, it usually means that the 'lighting designers' haven't finalized their vision yet, but we can't wait for them...we need to get this thing bid and awarded.
In order to keep things fair for the bidders, I give them a line item price for X. That way, it's possible to get a reasonable bid from everyone instead of a bunch of wildly differing assumptions about what X will finally turn out to be. It's a LOT easier than horsetrading later on for materials and labor when the Bid included 34 $200 lights but the final design called for 42 $160 lights.
If the final price of X is over the allowance, the Change Order is smiled upon with very few questions. If the final price of X is less, I usually let the unspent money go back to the Bidder.
Some folks may handle it differently; it would be safest to ask that before your bids are due.