We were using a mix of tools throughout Microsoft: several teams were using Source Depot, which is an internally developed centralized version control system. It was built to handle large teams, like Windows and Office. It was the precursor to Team Foundation Version Control, which is the centralized version control system available in TFS and VSTS, which is also capable of scaling to large projects, and many organizations within the company were (and are) using that.

GVFS is part of our effort within the company to standardize on a single set of best-of-breed tools, and use the same tools inside Microsoft that we deliver to customers. So we're adopting one engineering system throughout the company and we're moving everybody to Visual Studio Team Services.