The Source 2013 engine branch (also called SteamPipe branch) is the final iteration of the Orange Box engine branch. It was introduced by Valve in 2013 to replace the archaic GCF file format with the VPK format introduced in Left 4 Dead. Major shifts in content mounting occurred and also brought Half-Life: Source from the Source 2006 engine branch to this version. The "custom" folder was also added to make mounting easier and was added as a mounting path for each individual Source game.
While SteamPipe was praised for simplifying how the Source engine mounts content, and including previously-unreleased TF2 code, it also broke countless numbers of mods and worsened problems already present in the Source 2009 branch. It also does not have most of the features that the engine branches from Left 4 Dead onwards had.
Source 2013 Singleplayer
Formerly Source 2009
Gamecode for the 2013 singleplayer branch is included in Valve's Source SDK 2013 repository.
Source 2013 Multiplayer
Formerly Source MP
Gamecode for the 2013 multiplayer branch is included in Valve's Source SDK 2013 repository.