Some clients have "app relays" because they haven't implemented the outbox/inbox model yet and are just talking to these specific app relays and none other, some use these "app relays" as fallbacks for when they can't find anything using outbox and other heuristics, like Snort. Some clients don't have the concept of app relays, like Gossip and Coracle. Some apps have search relays in a separate category, like Yana and Amethyst.