A token used for optimistic locking. AWS WAF returns a token to your get and
list requests, to mark the state of the entity at the time of the request.
To make changes to the entity associated with the token, you provide the
token to operations like update and delete. AWS WAF uses the token to ensure
that no changes have been made to the entity since you last retrieved it. If
a change has been made, the update fails with a
WAFOptimisticLockException. If this happens, perform another
get, and use the new token returned by that operation.