It actually represents a sliding space between the thoracic wall (the costal grill) and the anterior surface of the scapula.
A serous bursa allows this sliding.
The scapulo-thoracic mobilities are related to the sliding and rocking of the body of the scapula on thorax.
The scapula remains ‘plated’ in the thorax via the anterior serrated muscle (innervated by the long thoracic nerve).