what does COB mean dealing with health insurance billing?

what does COB mean dealing with health insurance billing? i was asked this question and i have no idea what this means the question is what is the "birthfday rule" that goes with a COB when dealing with a child when billing for a medical service?
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COB is coordination of benefits, which means that if a family is insured under more than one policy, then any payouts for benefits are shared/divided between the insurance agencies. The birthday rule is used to determine who is the primary insurer of a child whose parents each have an insurance policy that could cover the child. Under the birthday rule", the health plan of the parent whose birthday comes first in the calendar year is designated as the primary plan. Note that it doesn't matter which parent is older, because the year of birth is not a factor. Thus, if your birthday is July 15, 1955, and your spouse's is Sept. 17, 1953, your health plan would be considered primary because your birthday comes first in the calendar year. This is supposedly a "fair" way of determining which policy is the primary one when more than one plan is in effect.
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