Bungie is moving from Activision Blizzard’s Battle.Net launcher over to Steam on October 1 (a delay from the original September 17 date), but before then players can link their PC accounts and saves.
On August 20 you can link your PC player accounts to start transitioning your Guardians and purchases, and the next day on August 21 Cross Save launches. Cross Save enables players to select an active group of characters and play them on any connected accounts across different platforms.
Bungie is coordinating this migration process with not only the move from Battle.Net to Steam, but also with the Shadowkeep and New Light additions. Their release dates were recently moved from September 17 to October 1.
[Source: Bungie]
