Data Modification Commands with readpast and Isolation Levels

If the transaction isolation level for a session is 0, the delete, update, and writetext commands that use readpast do not issue warning messages.

For datapages-locked tables, these commands modify all rows on all pages that are not locked with incompatible locks.
For datarows-locked tables, the commands affect all rows that are not locked with incompatible locks.

If the transaction isolation level for a session is 3 (serializable reads), the delete, update, and writetext commands that use readpast automatically block when they encounter a row or page with an incompatible lock.

At transaction isolation level 2 (serializable reads), the delete, update, and writetext commands:

Modify all rows on all pages that are not locked with incompatible locks.
For datarows-locked tables, the commands affect all rows that are not locked with incompatible locks.