Chest Circumference is typically measured at or just above the line of the areola (i.e. nipple). It's generally used within the fitness industry to measure impact to certain muscle groups of an exercise program and the fashion industry when an indiduals body to assess clothing fit.
CompleteScan measures the largest girth of the chest - typically at or just above the line of the areola (i.e. nipple). Demonstrated below.
First raise arms to head to place tape.
Once correctly place, arms are dropped to 45 degrees.