Reviewers may abstain from reviewing any assigned submission. To allow reviewers to do this, add a special type of field to the reviewer form.
Only check box fields can be made into an abstain field.
This is because the front-end of the review site (where reviewers will see it) is set up such that only a reviewer who wants to abstain will check this box.
An example of the front-end view is below:

To configure Admin emails so the admin receives an email when a reviewer submits an abstention.