A transaction summary is a summary of how a transaction impacts a given transaction party's channel. A sender and recipient each have a different transaction summary as their channels are impacted differently: one is debited, the other is credited. A summary object is always required for a transaction to be considered completed, and it can be used to dispute a transaction. The purpose of this object is two-fold: first, we can now accommodate sophisticated transactions, without the need for the smart contract to interpret every aspect of them; and second, to act as a commit protocol on a transaction for the purpose of easy and consistent recovery from errors. Note that the sender (and in some cases the recipient) is required to complete a transaction, but a zone is allowed to fail any transaction it is a part of all by itself, but up to a point. When a transaction is in "Completed" state, it is final and may not subsequently fail or be "clawed back". In this case you will be able to see and validate the counterparty's transaction summary as well, as a proof of delivery for your transaction. Similarly, a "Failed" or "Fail Pending" transaction cannot subsequently result in assets changing ownership.