I'm sure I mentioned this at some point, but from my observations on the BB US operations:

It appears that they have an allocation for online pre-orders, and store stock.
If the online pre-orders sell out, then the only option is "In Store Pickup Only".
Around release date some online stock might re-appear due to cancellations or failed payment method, then back to "Store Pickup Only" status.
I'm not sure if they still do this, but they also used to ship directly from the stores to customers to fulfill online orders. I received a few of those last year, you couldn't miss them, they had a sticker that stated "Hold for shipping to Customer", but have not seen any for a while.
Once store stock is gone, it goes "Sold Out", but apart from a few exceptions, it's only awaiting replenishment in a smaller quantity which is mostly for online purchases.
Once the item is listed as "Sold Out" a second time, odds are they are gone for good.

Why am I writing this? Maleficent already Sold Out once and got restocked online. There is still online stock but stores are drying up pretty fast, no store has any stock within 250 miles of my selected store. My guess is that it will get Sold Out for good pretty soon, and the odds of finding any in stores will be very slim in a short while.

Speculation on my part obviously, but based on previous patterns.