This document discusses when and how to use multi-document transactions in MongoDB. It provides examples of use cases for transactions, such as processing related data across multiple collections atomically. It also describes characteristics of transactions, including all-or-nothing execution, snapshot isolation, and how write conflicts are handled by aborting transactions. The document provides best practices for transactions, such as committing read-only transactions quickly and aborting abandoned transactions to avoid WiredTiger cache pressure. It also notes that DDL operations require intent locks that pending transactions cannot commit until released.