Ever since Red Dead Redemption 2 marketing materials were sent out to retailers it was clear that Take-Two and Sony had huddled up real close. All the material was PlayStation 4 co-branded with text stating that Red Dead Redemption 2 is "best enjoyed on PS4".

Until now, we had no details about what this meant, but some kind of exclusive deal on content was apparent.
Red Dead Redemption 2 is still a multiplatform game releasing on both PlayStation 4 and Xbox One simultaneously on the 26th of October, with the timed-exclusivity deal referring to content for the game, rather than the base game itself. In today's reveal of the game's special editions, Rockstar included a telling statement:
PlayStation® Exclusive: Play select content first on PlayStation®4. Details to follow.
"Select content" is a very wide term, however Sony has some other timed-exclusivity deals with multiplatform titles that can serve as an example - most infamously, Destiny 2.
Destiny 2's major expansions, which fuel its games-as-a-service model and keep players coming back for more, are released on PS4 first, leaving Xbox and PC players to wait for the content that keeps the game worth playing.
Beyond dividing the community and sparking animosity towards developers and publishers from less-informed players who don't know that console manufacturers strike these deals, timed-exclusivity likely achieves little. The vast majority of mainstream and casual gamers have one console and they won't go out and buy another system just to play DLC a week sooner.
People who own both consoles and will have their choice of which one to buy a game for swayed by timed exclusivity is, I guess, a very small demographic in the grand scheme of things. But then publishers and console manufacturers keep making these agreements so there must be some statistics backing up the business viability.
Red Dead Redemption 2's post-launch content support is something we know absolutely nothing about at this point and all pre-order bonuses have been confirmed to be identical on both platforms the game is being released on. Whether the 'select content' applies to minor item packs, GTA Online-style multiplayer DLC or full-blown expansions is anyone's guess.
Red Dead Redemption 2 will be released on Xbox One and best enjoyed on PlayStation 4 on the 26th of October, 2018.