Dental insurance: Benefit Limit and 50% co-insurance. Explain?

I have dental insurance which states that for "Routine Dental Treatment" that a "50% Co-insurance applies". There is then "Benefit Limit" of $400. Can some please explain what this means and how the Benefit limit and 50% Co-insurance are related. Thanks!
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They pay $400 or 1/2 of the cost, whichever is less. You pay the rest. For example: a) If you need $600 of care, then they pay $300 and you pay $300. b) If you need $1000 of of care, then they pay $400 and you pay $600.
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