Area Instance Versioning 

This property defines whether there can be only one or several area instance versions of an area instance. Without versioning, there is only one version and the area instance is equivalent to this area instance version. With versioning, there can be several versions in different states, and an area instance is the quantity of all area instance versions with the same area instance name.

Without versioning, several readers from different internal sessions can access an area instance after it has been built; a changer can access it only if there are no read locks at that time. With versioning, a change lock can be set on an area instance that still has read locks.

The maximum number of versions of an area instance can be specified in the runtime-dependent area properties.

...