Backwards Compatibility on Xbox One has been received with praise since Microsoft dropped the bomb at E3 a week ago. Plenty of details rolled out post-E3, but a few continued to be elusive: Multi-Disc, DLC, and Kinect support... until now that is.
Phil Spencer sat down with Giant Bomb on their E3 Podcast not that long ago to talk about how Backwards Compatibility works, and clear confusion on some of what it can & can't support. He went on to say that multi-disc 360 games, like Wolfenstein: The New Order or Dead Space 3, are currently in development, and are being looked into. He said specifically, "We're still working on multi-disc. Lost Odyssey, and Blue Dragon are some of my favorites from the 360. There's actually work in packing a multi-disc into single that requires us to go back and look at the original package on the multiple discs and reconfigure that. There weren't allot of multi-discs on 360, but that is an issue for us."
He also made a brief comment regarding Kinect: noting that 360 Kinect games will not work on Xbox One, "There's some caveats, and as always I like to be as transparent as I can be on this: Kinect games won't work from the 360, because translating between the Kinect sensors is almost impossible."
Lastly, Xbox Support on Twitter confirmed that disc-based & digital 360 DLC will be supported, and can be transferred to Xbox One via the Backwards Compat. system. However, the availability of the DLC depends on whether or not the Developer or Publisher approves it.
For example, if a company like ZeniMax were to allow it's consumers to port it's 360 DLC to the One, content such as Dawnguard from Skyrim & Mothership Zeta from Fallout 3 will be playable with no strings attached (the content still needs to be tested beforehand, like Phil Spencer said in the podcast above), and possibly all DLC from the GOTY and special editions of said games will be utilized as well. Again, this still depends on the content-holders giving the green-light.
Backwards Compatibility will be available to everyone later this year, and supposedly will launch with around 100 supported titles, and hundreds more every month. Those who are in the Xbox One Preview Program can test the system's current titles right now, like Perfect Dark Zero, Super Meat Boy, and Battleblock Theater.
Xbox additionally has a public vote on which games will be approved. You can go to the Xbox Feedback forums here to cast your vote on a game you would like to see on Xbox One.
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Halo 5: Guardians has plenty of pre-order bonuses that are acquired from different retailers, such as Agent Locke's HUNTER armor from Gamestop & GAME, the Projection SMG from Amazon, and a loadout weapon & skin from Xbox Live. The incentives to buy the game from all these retailers makes the content too exclusive for any one person to get unless you want to go through a large amount of hassle. It makes the choice & lack of choice of place to preorder all the more difficult.
Relieving the pressure on fan's shoulders, Josh Holmes of 343 Industries made a quick response to a user on Twitter regarding availability of the preorder content. He confirmed that all preorder content can be unlocked in the base game regardless of who you bought the game from, and that the preorder itself just "- guarantees you have it out of box.".
So earning something like the Sentinel Battle Rifle from Gamestop will require work in-game to get, but you'll still have a guarantee of the content being unlockable. 
Thanks for reading! 
Sources:
Xbox Support on Twitter
Josh Holmes on Twitter
Giant Bomb's E3 Day 1 Podcast
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