Extent (Index Keyword)
To specify that this is an extent index, use the following syntax:
Index State [ Extent ];
Otherwise, omit this keyword or place the word Not immediately before the keyword.
If you are using bitmap indices, then an extent index is automatically maintained and there is no need to define one.
An extent index is used to keep track of which object instances belong to a subclass.
The Default for the Extent keyword is false.