The mechanism Oracle actually uses to track the free space in an extent is the freelist.
Each table has one or more freelists, which contain a list of all free data blocks in the current extent.
If a data block is filled to the maximum space allowed, it is removed from the freelist. If the space used by a data block drops below the percentage specified in the PCTUSED parameter, the block is placed back onto the freelist.
You can specify more than one freelist for a table. Because Oracle must check the freelist for each insert into a table, you can have multiple freelists if your usage scenario calls for frequent simultaneous inserts. Multiple freelists can avoid any potential contention for the freelist contents.