However, if the extent is not allocated to a specific instance, then the blocks themselves are allocated only when the high water mark moves.
When the existing extents of a segment are full, Oracle allocates another extent for that segment.
The next level of logical database space is an extent.
An extent is a specific number of contiguous data blocks allocated for storing a specific type of information.
Free space is allocated for insertion of new rows and for updates to rows that require additional space (for example, when a trailing null is updated to a nonnull value).
In data blocks allocated for the data segment of a table or cluster, or for the index segment of an index, free space can also hold transaction entries.
Some block overhead is fixed in size; the total block overhead size is variable.