CVS Tag Naming Convention
pXXX is a branch.
bXXX is only an interim build number. ie a tag only. It is something we use for internal QA when we are testing.
p300 is the maintenance branch. This is the branch that all 3.0 specific fixes will be made to. It will change over time. It will remain stable and for folks developing to the 3.0 release this is what you should be using.
milestone_3-0-0 is the spcific tag for the 3.0 milestone release. Its a moment in time and will not change.
HEAD is a special tag name for the very latest code base. This is normally less stable than a maintenance branch and should really only be used if you are working with new features and the like. For example, the next major release 3.1 will be developed on this.