[Touch Diamond, MDA Compact IV ROM Development] awran5:
here is some info about Differences between branchs :

Microsoft has different COM trees which are active. All those branches / trees focus on a certain aspect of the OS and remain active even after the OS has been released.

For the 6.x build there are 5 active COM trees. The oldest (COM1) is the active development tree for 6.1, but it does not spawn many new builds as the focus is now WM 6.5.

COM1- Builds 217xx:

is the active development tree for 6.1.

COM2 - Builds 218xx:

GUI ( Graphical user interface ) is 1st stage of finger friendly features implemented. This is the only official WM6.5 released to public. All other COMx Builds are leaked test builds (not even classified as alpha or beta). In this range, the GUI introduces softer corners to various dialogs, the new start menu (Very early builds had a honeycomb theme but although the hexagons have been removed, the icon alignments remain), and the taskbar and menubar have been slightly modified to allow for further skinning.

COM3 - Builds 230xx:

from 23001 to 23021 looks the same as COM2 and share the same base. after build 23017 or 23022+ (Not Sure) introduced the new shorter/expanding taskbar and finger friendly text-based buttons on taller bottom menubar and ever since its been called wm6.5.1 rather than 6.5..

23047+ introduced icon-based buttons.. New checkboxes and scroll bars and recently (from 23069 i Guess) a magnifier in the taskbar are amongst the newer gui elements introduced in the latest builds..

It appears that COM4 elements are being introduced into this branch such as the new contact application, a slightly modified sms application, and possibly a slightly modified calender application..

This build is very experimental can contain bugs. and i've read it will probably be terminated and integrated into COM5 or now COM8.

COM4 - Builds 234xx:

It is known for it's new Outlook interface, and The Outlook updates have been coorporated into the COM3 branch. it Seems to have stopped at 23420. The newer contact application and possibly the modified sms and calender applications were first introduced in this branch but have since been incoorperated into latest builds of COM3

list of changes : [Video]

- Email folder in Microsoft Office Outlook Mobile gets new colored icons.

- Improved browsing experience on Internet Explorer Mobile.

- Definitely faster and more responsive than the last leaked Build 23053.

- Faster boot time.

- Build 23420 has the new contacts app.

- Build 23060 has the menu eye candy but not the new contacts app.

- Incorporates the updated GUI from the later COM3 builds into COM4.

- Better memory management.

COM5 - Builds 235xx:

starting with 235xx, seems to have been compiled at about the same time as 23060 but with superior stability, speed, memory management, Newer COM3 builds 23071 and above have been reported to be superior in terms of stability and speed. However the latest COM5 23506 compared to COM3 23088 (build date is identical) seem equally fast and stable. COM5 builds have the new Contact app and Magnifier, as in latest COM3 Builds, but lack kinetic scrolling.

In the meantime, we should mention that the list of changes the new build supposedly comes with also includes:

- Bottom taskbar color is changed to Vista-like dark blue.

- Some GUI fixes for Volume Control.

- Graphically enhanced Scrollbars and checkboxes.

- Graphically enhanced buttons throughout the operating system.

- New and improved Contacts app which was first seen in Build 23412 (COM3 Branch). [Video]

Other branches you might come to see :

WMD - Builds 28xxx:

The First Build leaked is 28002 (build date 17th November 2009). As of this branch Windows Mobile is at WM6.5.3. This branch is to continue from COM3 23090, aslo shows a new on-screen keyboard, better suited for finger operations. Making Windows Mobile more touch-friendly involves a new soft keyboard, as well as changes with the menus, Start button and top and bottom bars. Since these changes have been already spotted, we should wait to see what the final version will actually look like.

leaked builds that have been played with by the team over at:

213xx = MOT Motorola

215xx = SAM Samsung

216xx = HTC

219xx = MD, feature branch (unstable features are added here, backported

22xxx = SEMC Sony Ericson

236xx = LG Electronics.

This info has been taken from many posts so credit goes to the original author​