In it says:

NOTE: Should discuss how branches are synchronized using the MERGE-* tags.

Could someone give me some hints about what exactly is meant by this?

NOTE: Emphasize that before merging a branch with newer base code, all recent OpenSSI/CI changes to that branch should first be synced to all other branches. Otherwise, the recent OpenSSI/CI changes are lost in the "noise" of the base changes.

And this?