A more thorough explanation:

A reviewable issue is a decision or ruling that can be appealed or reviewed by a higher authority. It is a decree or order that is considered final enough to receive appellate review or an interlocutory decree or order that is immediately appealable by statute.

• An order granting summary judgment
• An order denying immunity to a police officer in a civil-rights suit

These examples illustrate how a reviewable issue can be a decision made by a court or agency that can be challenged or appealed by a higher court or authority. In the first example, a party may appeal the decision to grant summary judgment if they believe there are still factual disputes that need to be resolved. In the second example, a police officer may appeal the decision denying immunity if they believe they are entitled to immunity under the law.