To get a record lock of sufficient strength for an operation, you must first have a table lock of sufficient strength. Regardless of the current transaction isolation level, if the application's intent is to perform an operation other than a fetch, the lock mode is strengthened for the operation.

With the exception of the READ UNCOMMITTED isolation level, you are not prohibited from creating or updating records based on the transaction isolation level. It is the responsibility of the RDBMS to provide sufficient lock escalation when an operation is being performed that requires lock upgrades. Note that you are prohibited from creating or updating records when the isolation level is READ UNCOMMITTED.