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.
COM2 and COM3 share the same base, but after build 23017 the COM3 tree branched off and got the 'new' layout. At this point COM2 remained the active development tree for 6.5 and spawns updates regularly (fixes the current released 6.5 build of Windows Mobile).
COM3 up to build 23088 is the tryout build for the new layout of Windows Mobile (replaces start menu, dropdown taskbar, etc.). This build is very experimental can contain bugs. It will be around for another couple of weeks when it will probably be terminated and integrated into COM5 or now COM8.
COM4 the branch, 234xx, is the (in)active OS development branch. It focusses on updating the main Windows OS (Win CE 5.2). It is known for it's new Outlook interface. It's currently uknown (to me) if it's still active, but it hasn't got any updates in the past few weeks ago. The Outlook updates have been coorporated into the COM3 branch.
COM5 is the branch starting with 235xx, and contains at this point nothing new. It's unknown what the main focus is, but it is likely to replace COM3 and COM4 and integrate all the changes uptill now. This means the other COM trees will die off and become active once the OS get's released (as was the case with COM2 and COM3)...
COM8 is currently the most stable and contain the integration of another experimental branches.