I can't speak to Amanda Steinke's use case but I have run into this before as well. Product definition is not always start with low level parts and build up... sometimes it's start with an off-the-shelf product (probably something that you have purchased) and remove some stuff, add other stuff thereby transforming it into a new solution/deliverable. For these use cases, IMO what comes off is as important as what goes on. In order to validate accuracy things need to be rationalized... did all of my add items get consumed and do I have all of the deduct items remaining?
To resolve this I have previously used a custom unit of "dea" (for deduct each) to represent the negative. Then 1 dea = -1 ea, but this was representative only with no systematic propagation of BoM information to an external system such as ERP/MRP. The same challenge holds for fractional or decimal units. I won't say all ERP/MRP systems do this, but some that I have seen allow for both negative and partial quantities associated with each. It would be nice to be able to do the same in Windchill so that unit interpretation did not need to be applied at data propagation.
As far as CAD-driven Part Structures goes, that's a whole different can of worms. CAD allows for flexible parts and bulk items, both of which may be best expressed in non-each quantities, yet Windchill seems to only see CAD in unit (each) terms. The deduct use case might apply if you were using a process assembly and suppressing a component in a later step - I have not tried this (even to see if Creo allows step specific suppression and if so does application behavior align with design intent). It would be ideal if it did as it would allow for complete and robust definition.
