A MutatingAdmissionPolicy must already exist in the cluster with no binding, so it is dormant and not enforced.
Create a MutatingAdmissionPolicyBinding naming it, which matches cluster-wide unless matchResources narrows it.
The policy takes effect on the next CREATE.
What it does is whatever that policy already specifies, which may be harmless.
Note:
Binding write is not a safe reduction from authoring.
Create on mutatingadmissionpolicybindings alone activates a dormant policy with no rights over the policy itself.
Severity is whatever the existing policy already does.