Co-insurance in the health insurance context is a little different from, say, your regular commercial property insurance.
The co-insurance requirement for commercial property insurance is a requirement to insure a large enough portion of the value of the property. For example, if your property is worth $1,000,000 and you have an 80% co-insurance requirement, then you are required to pick a limit of insurance of at least $800,000. If you don’t and a loss occurs, you would be penalized for not meeting the co-insurance requirement.
With health insurance, the definition of co-insurance is a bit more literal. It just means the percentage of covered healthcare services you are required to pay out of pocket on top of the deductible you already pay. For example, let’s say you have a claim valued at $11,000 with a $1,000 deductible and a 30% co-insurance clause. In this case, you would pay out of pocket the $1,000 deductible plus 30% of the remaining $10,000, equaling $4,000 in total (learn more in All the Ways You Pay: Premiums, Deductibles, Co-pays, and Coinsurance).
Of course, this is subject to your out of pocket maximum. If the amount calculated above is greater than your out of pocket maximum, you would only be required to pay the maximum.