No, the linear measurement means how long of a distance the water can flow over.
Imagine a waterfall. It is 10 meters across. This is the type of measurement you want.
Now for your overflow, say you have a standard corner overflow, you have two lengths you have to add up for the total linear measurement.
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It's for drains. So that would probably mean "free flowing".
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"Probably" is not a good term in plumging a reef.
Leads to too many floods. That is why this calculator is dangerous. Good as a gernal guide though, a starting point better than nothing.
If you have 20 inches of water in your overflow pushing down on the drain, it is going to flow faster than if you have 5 inches of pressure.
Also you can get some suction/siphoning going, but that is not recommended.