When a repeater is built, you receive build points for 2 buildables. These build points are available for that repeater only, and are not available for any other repeater or the reactor. That's what is meant by zone building. Essentially there is a zone around the repeater and the ractor in which you can build stuff and that zone has buildpoints specefic to it.
Also, when the enemy destroys one of your buildings, it takes time for the build points to return.