Under a holdings record in the Instance details view, the Inventory UI currently displays Items directly associated with a holdings record by foreign key (in a many to one relation that is).
With the introduction of bound-withs an Item can reversely be a gathering of many holdings records (and thus many titles). The Inventory UI needs display these bound-with Items too below each of the holdings records that are part of the bound-with even if the Item is not directly related to the holdings record by a foreign key.
For this, a new back-end API is required, that will find and retrieve an Item, that represents a bound-with, by the holdings record ID of one of the parts.
(the pieces that are involved in bound-withs are marked with a "(B)", see also