The main purpose of releasing the Forge editor is to give Halo 5 map makers some more options in creating multiplayer levels for the Xbox One first-person shooter. However, as Ars Technica confirmed with Microsoft, there will be a nice side-effect:
Microsoft representatives have confirmed to Ars that the free Windows 10 game will support unfettered online play with anyone on a player's friends list. That means players can create or download a Forge map and invite anyone else playing the Windows 10 version to join in and play to whatever "kill count," time limit, or other win condition they've set. Even better, Microsoft says that this friends-only multiplayer mode in Windows 10 will fully support mouse-and-keyboard game controls.
That's the good news. The bad news is that this release does not mean that Halo 5's single player campaign, nor any of its non-Forge multiplayer maps, will be coming to Windows 10. It also won't bring over any of the game's matchmaking features, nor does there appear to be any plans to support cross-platform play between Windows 10 and Xbox One.
Many years ago, Microsoft did port both Halo 1 and 2 to PCs, but none of the other Xbox games in the first person shooter series have ever made it to the PC platform. In March 2015, Microsoft, in collaboration with Saber Interactive and Innova Systems, announced that closed beta testing of the free-to-play multiplayer game Halo Online had started in Russia for the PC. However, that test was "temporarily suspended" in December 2015, and there's no word on when, or if, it will resume.