Already posted on my blog but since no one ever reads that I'll post it here.

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If you like playing with beta versions of Windows, you'll have seen that strange looking version number.
If you've asked yourself what it means, here it is.

Here's an example build tag:
6.2.8102.101.x86fre.winmain_win8m3.110830-1739.92eb4451821f0730

So let's break the full tag into several smaller parts.

• NT Major Version
• NT Minor Version
• Build number
• Revision
• CPU Architecture/Build Type
• Build lab
• Compilation date
• Time of compilation
• Rights Account Certificate GUID1

1: Present only in Windows 8 M2 builds and up

NT Major Version:
This is the first section of the build tag. In this case, it's 6, showing that that the code is NT Version 6.
It's changed when major overhauls of the NT code have been done.

NT Minor Version:
This is the second section of the build tag. In this case, it's 2, showing that this is the second minor release of NT Version 6.

Build number:
This is the third section of the build tag and in most cases, where Winver stops (for example, Version 6.2 (Build 7955)).
The build number shows how many times the NT code has been compiled since 1988 or so.

Revision/Delta:
It shows how many times little tweaks and bugfixes have been made. These changes aren't significant enough to warrant a full recompile of the code, but they are still recorded.
Inside Microsoft, this section is known as the Delta.2
2: Thanks to Daniel for this information

CPU Architecture/Build Type:
What type of CPU the build was compiled for. A few different ones exist, including x86 and AMD64.
The Build Type is how it was designed to be used. There are two different Build Types: Checked and Free.
The Free build is probably what you're on right now. All code optimizations are present and if a critical error occurs, the OS traps it.
The Checked build lacks these optimizations however and is much slower than the Free build. It also depends on a debugger to be attached for when a critical error occurs.

Build lab:
Inside Microsoft, there are many different groups working on the NT source code. Things would get pretty confusing if this section didn't exist, so each group (called a lab) has its own name.
For example: winmain means the code was compiled in the Windows Main build lab. winmain is where all the code from all the other build labs gets stitched together for the next winmain release.
Sometimes winmain may be followed by _win(version)mX; this is the Milestone part of the tag. Milestones are goals the Windows developers are working to.
This Milestone part of the tag might be seen as winmain_win8m3.
Other build labs that are commonly seen are the FBLs, or Feature Build Labs. They write the code to all the fun stuff in Windows.

Compilation date:
This is the date this build was compiled. It's read as YY/MM/DD.
A build tag with 110228 as the compilation date would mean the build was compiled on February 28th, 2011.

Time of compilation:
The last part of the build tag.3 It's in 24-hour format, so 1930 would be read as 19:30 or 7:30 PM.
3: Only the last part of the build tag in Windows builds prior to Windows 8 M2

Rights Account Certificate GUID:
This part of the tag is present in Windows 8 M2 and up.
It's used to enable locked features on a per-employee basis and is unique for every key provided and for every build compiled., making it easy to trace the build unless the default product key has been used.
It is tied to several licensing system components and the hash is generated by an algorithm that is made by several objects within the core Windows files.4
4: Thanks to arseny92 for explaining how this works and what its name is.

Review:
The build tag 6.2.8102.101.x86fre.winmain_win8m3.110830-1739.92eb4451821f0730 means this when broken down:

6 <-- Major version of Windows
2 <-- Minor version of Windows
101 <-- Revision
x86fre <-- CPU Arch. and Build Type
winmain_win8m3 <-- Build lab
110830 <-- Compilation Date in YY/MM/DD
1739 <-- Time of compilation
92eb4451821f0730 <-- Rights Account Certificate GUID (Win8 M2 and up only)

deleted post, problem solved.

So, Neptune was compiled later than XP RTM?
Are you sure that 7599 builds had been compiled before Windows 7 RTM?

So you're right. Fixed.


I really have no idea why Neptune started in the 5xxx range.

If not, very close to that many.

There was some discussion that Windows build's numeration isn't continuous but discrete.

It's known that they decided to jump builds for public release, but what isn't known is whether or not they still keep compiling behind the scenes after RTM to see what else they can do to their code.
Post-RTM compiles are known to exist of 7, with build tags of around the 77xx range.

Probably much more than 7600 times, just think about all the different build labs and recompiles...

Why doesn't this thread get stickied? It's a helpful post for new members or existing members like me who've never paid attention to it.

some usefull info, thanks!

Thanks for this awesome tutorial, yet I knew about some stuff

Linuxlove for mod!

Well that's the second person who suggested I should be a moderator.

It's great to have all of this stuff compiled into one location. I can't count the number of times I've been reading an article and wanted to refer to something like this.

Andy and some others would have to decide about a mod but what about a new title called "Tutorial Master"?

lol, and i always thought fre after architecture means French...

Nice post, but for clarity I would change 'the first minor release' to 'the first minor modification' or something like that, to avoid ambiguity.

Do we know when MS started using build tags?

During Whistler I think.

Thanks!
I think it will be usefull soon.

interesting. I knew it meant something. but that's nice.

Sorry, I need to step in here. The 74xx and 75xx were built after RTM. A 72xx/73xx build became RTM. With the Win7 branch, there was alot of sub-branches and they made several post-RTM builds that were not built around SP1

Thanks for the info buddy. I appreciate your work to create this useful tutorial.

First leaked build with a tag is whistler 2202.

Nice post.

About the Build Number, I think that MS changes the build number only for a major release, and it actually counts how many times that major build was recompiled.

For example:
- Windows Vista went up to 6.0.6000 when the RTM was released and for SP 1 & 2 they only changed the last number of the build 600x .
- Windows 7 continued from the Vista build number and went even higher to version 6.1.7600 , as for Win 7 SP1 goes the same rule as for Vista SP's
- Now for the next Windows release , it started based on some POST-RTM releases of Win 7 (77xx) with some modification in M1 (6.1.7850) after wich the minor version was changed from 1 to 2 bringing the latest Mileston (3) to somewhere around 6.2.79xx.

Small addition: That revision number is called "(build) delta" by Microsoft...

Hm, looks like useful information for new users. Now, I think I'll go try it out now on some beta build I have