Transactions are grouped in so called transaction groups. The grouping of transactions should help you to get a better and more detailed understanding of your conversion rate.
Transaction are grouped by the customer ID. In case the customer ID is not provided (NULL) then each transaction is
treated as a separate transaction group.
A transaction group is considered as successful once a transaction is successfully completed.
A transaction group is considered as failed and closed once the last transaction is older than the transaction interval minus the actual time.