Scalar DB provides an ACID transaction on Cosmos DB. A "transaction" of Cosmos DB cannot operate items over multiple partitions or multiple containers. A transaction with Scalar DB can operate them on Cosmos DB.
engine in Azure Cosmos DB supports full ACID (Atomicity, Consistency, Isolation, Durability) compliant transactions with snapshot isolation. All the database operations within the scope of a container's logical partition are transactionally executed within the database engine that is hosted by the replica of the partition. 4 https://docs.microsoft.com/en-us/azure/cosmos-db/database-transactions-optimistic-concurrency