The ratio units are FTE units or equivalent employees working full-time. In other words, one FTE is equivalent to one employee working full-time.
For example: You have three employees and they work 50 hours, 40 hours, and 10 hours per week - totaling 100 hours. Assuming a full-time employee works 40 hours per week, your full time equivalent calculation is 100 hours divided by 40 hours, or 2.5 FTE.