Check if the quality of wild pets in a battle is better than pets of the same species in your journal. When the addon detects a pet with better quality, it displays a message in the default chat frame. The same happens when it detects a non-collected pet.
When a new pet is captured the addon will check if you have any pets of the same type and notify you if you have three pets of the same species where a least one has lower quality than rare so you can release one to make room for more rare pets.
To create a complete Pet-Battle experience, also try these add-ons: