What does it really mean when my dog exposes his belly like this?

This is moose, he’s my 2 year old german pix (pyranees/anatolian). He’s the sweetest dog my families ever had. Whenever you come home, he gets so excited that he grabs the nearest shoe/boot and carries it around while following you.

Sometimes when he sees me, he does this thing we’re not only does he expose his belly, but he will lay flat on his back and put his legs up in the air.

What exactly is this behavior? I want to say that it’s content/invitation to belly rubs. But at the same time, could it be some time of submission??