Crossplay will be available for multiplayer between the various PC versions of Halo: Reach (Steam and Microsoft Store/Windows10). We will not support Xbox/PC crossplay at launch, but this is something the team will monitor and evaluate over time. If you own both the Xbox One and PC version, your campaign progress will be unlocked and shared on each platform as progress is made. Please note that mid-mission saves will not transfer between PC and console builds as they are not compatible.
Below is a handy graphic that explains Crossplay & Cross-Progression support for Halo: Reach and MCC.