i'm not sure that I totally understand your relationships, but let me guess at an answer.
When you select a main category to then drive the filters for the next level of selection of Sub category, you should rename that like [Main category filter], as opposed to [Main Category]. Then use a lookup to get the [Main Category] as a lookup from the Inventory record. Same with the Sub Category.
The point is that if you Change the Main category from the Master Inventory record, you want tho flow down though the lookups. So differentiate between the data entry filter you are using and the live lookup values which may change if the master changes.
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Mark Shnier (YQC)
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