9:11 AM, Thursday December 15th 2022
The plane you highlighted above the eyes is the forehead plane. It connects to the top edge of the eye sockets and will almost always be angular. In some animals (like the rhino linked above) it may extend a way off the cranial ball at the top. This is part of taking the round cranial ball and transforming it into a more planar construction. We're still keeping things simple, but these planes are closer to the structure of a head than the ball we started with. We're using planes rather than rounded masses up here because this area is usually pretty hard and bony (tap your own forehead, it's hard, no?) Uncomfortable draws it in this specific way- attached to the top of the eye sockets, as a way of connecting all these puzzle pieces together, so nothing is floating, everything fits together.
The other section you highlighted is the cheek, and this one varies a lot more. You'll need to study your reference to see what size and shape you need for that particular animal. Hopefully you can see that this piece is also wedged against the other parts we've already drawn- the eye socket and the muzzle.
Here are some further examples of the informal head demo method in action:
These notes Uncomfortable made on another student's work may help you understand additional masses. That image also shows how to start your head constructions.
Hope that helps.