A complex is a piece of property containing multiple buildings. Many Places in MB are parts of complexes of varying sizes and variety.

Example of a complex in MB: Greensboro Coliseum Complex which houses Greensboro Coliseum and White Oak Amphitheatre

I would also add a guideline something like this: Complexes primarily exist in MusicBrainz to link together associated Places located on their grounds. As such, all entities (Releases, Recordings or Events) should be linked to the specific Place at which they occurred. Complexes should ONLY be linked to entities if no more specific information about the venue can be found.