Locks 
The isolation of transactions is an important aspect when programming database-based applications. Isolation makes sure that two transactions that are running simultaneously do not affect each other. The technical realization of this is based on setting locks on read or changed rows in a database table. Databases provide multiple isolation levels, which determine how strong the isolation is between two transactions that are running simultaneously. Experience has shown that if the isolation is stronger, throughput and scalability decrease. Since the databases that are supported by SAP do not offer uniform semantics for locks, SAP does not use database locks to perform isolation, instead it uses logical locks. Logical locks are provided on the enqueue server. For this purpose, the database is operated on a low (weak) isolation level.
For more information, see:
Enqueue
Service