MVCI Weeks and MVCD Points inventory is kept separate according to the terms of each system's governing documents in order to protect Owners/Members ownership and usage rights. Inventory available through the DC Exchange Company is sourced from Marriott-controlled deposits as well as (according to a no-longer-available FAQ that had been posted to the owners' website during the early DC years,) "... other Marriott Vacation Club Owners who enroll their weeks and elect Vacation Club Points, and non-enrolled Owners who trade their usage for Marriott Rewards points or exchange their week through membership in Interval International."
Technically, in compliance with the governing documents, DC Trust Members have direct access to inventory in the DC Trust as well as inventory available through the DC Exchange Company; and, DC Exchange Members have direct access to only the inventory that's available through the DC Exchange Company. Functionally, it appears that Marriott is managing inventory such that a few select high-demand intervals are available only from the Trust at the 13-months Reservation Window, then at the 12-months Reservation Window most intervals are made available through the Exchange Company. Effectively, it appears that inventory is currently being managed by Marriott such that the technical v. functional legal aspects are practically nullified with respect to the overwhelming majority of available intervals.