A 15 mm Ross aortic retractor is used in cardiothoracic surgery, particularly the Ross procedure for aortic valve replacement, to hold back the aorta and provide a clearer view and better access to the heart and mediastinum for the surgeon to perform the procedure effectively and safely. The "15 mm" designation refers to the size or width of the retractor's blade, indicating it is a specific, smaller size used for precise manipulation of the aorta during certain complex cardiac operations.