Yes that is a "front access" skimmer and is often used for overflow into pools. Notice there is no lid, so baskets are a hassle. Some come with them, others don't. We remove them if they do because it's problematic as an overflow.
Shared equipment systems are designed to have spillover from the spa to pool, and the presence of that skimmer is indicative that this is the intended behavior of your system. Furthermore:
This makes sense because if you have your system in spa mode all winter, there is no water passing through this overflow into your pool. When the system is in Pool mode, the spa will receive a token of water from your pool and will overflow into the pool, keeping your spa and your skimmer fresh.
In spa mode, you system is only pulling from your main drain.
Can you send a photo of your equipment pad and your pool and spa from a macro perspective?