Hello Miranda, when you describe bom A and bom B, are you referring to two different parts that have their own BOM (user A should see part A and its bom and user B should see part B and its bom)? Or are describing a part that has a BOM and you want user A to see some parts in the BOM and user B to see to see other parts in the BOM? If it's the first scenario, then in addition to MAC policies, you could also solve it with Teams access control (Team A and Team B) and setting the appropriate team on the item.
You also have option to use role-based permissions using the managed_by or owned_by item properties in conjunction with the Owner and Manager identities giving access on the permission. Use a permission where the Manager identity has the ability to Edit. Create two identity groups (Group A and Group B). Populate the identities with user A and user B respectively.
-Dennis