From Blue Mars Developer Guidebook
Glossary of Blue Mars and general graphics terms
Block - An area within a city which can be exported as a file and edited by someone other than the city developer using the Block Editor program. Defined by an AreaShape which has an ARBlock entity applied to it. Stored as two layers in the city file (one for exported block, and one for imported additions).
Collision Proxy - A geometric shape used to determine when other objects or avatars collide with the given object and therefore stop or bounce off. By default the proxy is a bounding box, or it can be an added geometric shape created in a 3D program. Proxies are normally simplified relative to the main object geometry. This reduces the physics workload for the graphics engine.