Build lab

A lab or a VBL (virtual build lab) commonly refers to a team working on a part of the Windows operating system, or the source code branch that the team works on. Originally, a build lab was a room with machines periodically producing official builds of Windows NT. Initially, there was a single build lab for the entire project, however, as the number of developers grew, teams working on different parts of the operating system were assigned their own virtual build labs, which included a branch of the project code, as well as the infrastructure to build the code. This carried over to Source Depot branch names, which led to the term being also used for the teams themselves and the branches.

Builds that were not built by the build lab machines are called private builds. They are identified by having the  file flag set in the executable's version information and by including the account name of the person or service who initiated the build in the branch part of the build tag.

Labs
Microsoft has used several branching systems since the start of Windows XP development, which differ in the hierarchy of labs as well as in the naming scheme. However, there are some specifics common to most schemes, such as the presence of a top-most main branch, or the use of special branches for important development milestones.

Whistler and pre-reset Longhorn
The top-most branch (sometimes called the "trunk") was called main, which integrated changes from all labs. Under it were several numbered labs, each of which was working on a separate part of Windows, such as: These labs also had a "_N" branch, which served as a buffer between main and the actual branch. For example, changes from main would first get integrated into Lab06_N before later being integrated into Lab06 in a process called forward integration. The same applied for reverse integration, where the lab would first integrate changes into its N-branch before integrating them into main.
 * Lab01: Kernel
 * Lab02: Networking
 * Lab03: Server
 * Lab04: Terminal Services
 * Lab06: User interface
 * Lab07: Internet Information Services/COM+ (added in Longhorn)

There were also idx (internal development workstation/server) branches, builds from which are usually recompilations intended for TAP/OEM partners. However, they were also occasionally released for public testing, e.g. Windows Longhorn build 4074 or Windows XP build 2257.

Before the release of Windows XP, the source tree was forked into the xpclient branch while the main branch moved on to track Windows Server 2003 development. After the final version was shipped, new branches were created for updates, hotfixes and Service Pack development.

Longhorn through Threshold
The lab hierarchy was overhauled after the development reset of Longhorn to address the flaws that plagued most of the pre-reset period. Instead of having a small amount of general virtual build labs each focusing on a different general scope of Windows functionality, a new hierarchic model with considerably more feature branches has been introduced, which helped reduce the amount of code to reverse integrate for branch. Microsoft also set stricter criteria for reverse integrating changes from the labs into the trunk, which was now renamed to winmain.

During the development of Windows Vista, the vbl prefix standing for Virtual Build Lab was used for feature branches. The prefix has later been changed in the beginning of Windows 7 development to vbl, which stands for Feature Branch Level.

Notable Longhorn post-reset branches

 * vbl_core - kernel branch
 * vbl_ux - user experience
 * vbl_media_home - Windows Media Center