The mainline and branches can be changed as other projects in Matrix. The one thing which is different is, that if new items are generated they will have unique ids over all projects.
E.g. if you have in the mainline before the branch and MREQ-1, MREQ-2 and you create a new MREQ in the branch it will be MREQ-3. No matter where you will create the next MREQ, it will be MREQ-4. So you might have
Common Baseline (mainline at the time of the branch): MREQ-1, MREQ-2
Mainline: MREQ-1, MREQ-2, MREQ-4
Branch: MREQ-1, MREQ-2, MREQ-3