The image prefix is used for matching the image name against the image used in Enforcer. 

When you use a Docker image it has the prefix of the registry (usually a DNS name).

quay.io/mongo:latest is an image that is coming from "quay.io" registry. 

If you look inside Aqua UI - you don't see this prefix. This is because the image itself is saved inside the registry without the prefix. The prefix is only used by clients (Docker Client) to fetch an image from the right registry. 

So, you can add the registry prefix in order to sync between how the image is seen by Enforcer and how it is seen by Aqua Console.