Definition of ad hoc: Formed, arranged, or done for a particular purpose only. From Latin, meaning “for this.”
An owner (PI, preparer, contacts), a person with assigned read/write access, or an approver, can add an ad hoc reviewer to the approval flow. These manually-added reviewers can be for a particular role, such as a department reviewer, or can be a specific person.
Be sure, when you add a role, to check that ASTRA has at least one person designated as a reviewer for that node. Otherwise, your application could experience delays in routing.