Transaction Slot on a Block

Transaction Slot on a Oracle Block :

Your process does some DML that's going to update a table, so, Oracle reserves a slot in the transaction header of a rollback segment. This is specified by the rollback number, slot, and wrap. This is what's stored in the transaction slot of the data block that's to be modified. So, now that transaction slot in the data block points to that placeholder in the rollback segment header. When rows in the data block are updated, then in the data block's row directory, the lock byte for that row is set to point to that transaction slot. So, the row directory entry (lock byte) for an updated row points to a specific transaction slot in the data block's transaction header, which in turn points to the rollback transaction slot, which in turn points to where the before images are actually recorded in the rollback segment.

Reference :

Very good one...

http://www.proligence.com/itl_waits_demystified.html

Regards,

Navaneeth