If isolation level 0 (uncommitted) is specified, rows can be read without shared locks being requested, that is to say, shared locks are not implicitly requested. For this reason, there is no guarantee that a given row will still be in the same state when it is read again within the same transaction as when it was accessed earlier, since it may have been modified in the meantime by a concurrent transaction.
Furthermore, there is no guarantee that the state of a row that was read has already been recorded in the database using COMMIT WORK.
When rows are inserted, updated or deleted, implicit exclusive locks are assigned to the transaction for the rows concerned. These cannot be released until the end of the transaction.
Isolation level 0 is suitable for applications that use their own methods to enable concurrent access to objects, so guaranteeing concurrent access throughout the database system.