AsyncLevelOne means async delete the TSR after return.

AsyncLevelTwo means async ds.Commit() and delete the TSR after return.

AsyncLevelZero means no async commit.

DEFAULT sets the concurrent optimization level to 0, which means that when a write conflict occurs, one transaction will success.

PARALLELIZE_ON_PREPARE sets the concurrent optimization level to 2, which means there is NO guarantee that any transaction will success when a write conflict happens in different datastores.

PARALLELIZE_ON_UPDATE sets the concurrent optimization level to 1, which means there is NO guarantee that any transaction will success when a write conflict happens in a single datastore.