Oldest Interesting Transaction

To recognize which record versions can garbage collected, and which updates are rolled back and can be ignored, every transaction includes a "transaction mask" which records the states of all "interesting" transactions. A transaction is "interesting" to another transaction if it is concurrent - meaning that its updates are not committed, or if it rolled back - meaning that its updates should be discarded, or if it's in limbo.

The "transaction mask" is a snapshot of the states of all transactions from the oldest interesting, to the current. The snapshot is made when the transaction starts and is never updated. The snapshot depends on the number of transactions that have started since the oldest interesting transaction.

Taken from IBPhoenix