The unique identifier that all AutoCAD entities have that remains the same throughout the life of the drawing. Even if other entities are added or deleted and the drawing is saved, the Handle persists through the life of drawing. The handle is a 16-character Alphanumeric identifier.
The identifier that uniquely identifies data in a Database Table
A layer (or layers) that define where area outlines can be found.
A layer (or layers) that define where area identifiers (Room Name, etc.) can be found.