Data Blocks and Freelists (from ixora)[8]

each interested transaction list entry contains the following fields.

transaction id	8 bytes	this is comprised of the rollback segment number (2 bytes), the slot number in the transaction table of that rollback segment (2 bytes), and the number of times use of that transaction table has wrapped (4 bytes).
undo block address	8 bytes	the location of the undo for the most recent change to this block by this transaction. this is comprised of the dba of the rollback segment block (4 bytes), the sequence number (2 bytes), and the record number for the change in that undo block (1 byte), plus 1 unused byte.
flags	1 nibble	---- = transaction is active, or committed pending cleanout c--- = transaction has been committed and locks cleaned out -b-- = this undo record contains the undo for this itl entry --u- = transaction committed (maybe long ago); scn is an upper bound ---t = transaction was still active at block cleanout scn
locks	3 nibbles	the number of row-level locks held in the block by this transaction.
scn or free space credit	6 bytes	if the transaction has been cleaned out, this is the commit scn or an upper bound thereof. otherwise the leading two bytes contain the free space credit for the transaction - that is, the number of bytes freed in the block by the transaction.