Couchbase has introduced ACID transaction support through its SDKs starting from version 6.5, and later extended to N1QL in version 7.0, enabling multi-document transactions that can span multiple collections across different scopes within multiple buckets. This feature allows for infinite scalability while eliminating a single point of failure or contention. N1QL is a declarative language similar to SQL, used to insert, retrieve, and manipulate JSON documents. It supports ACID transactions in a single document but not before version 7.0, which introduced multi-document transaction support through N1QL. This feature simplifies application logic for complex applications and is useful in "System of Record" applications where multiple documents need to be updated atomically. Transactions can be used for debit-credit operations, logging custom actions that cannot be reverted, and calculating totals in many-to-many relationships. The use of N1QL transactions requires careful consideration of best practices, including limiting mutations within a transaction, avoiding delta table growth, and using implicit transactions for large updates.