Define "Protected Waters".

So many boats/Plans that I see say "For Protected Waters".

Does this mean extremely calm?......Does a lake qualify by virtue of it's
being a lake?....Lake Erie is a lake, and look how it eats boats.....

Here, around Houston, Texas, I have been to Lake Conroe, Lake Livingston,
Lake Somerville, Lake Travis, Canyon Lake...and so on......all of these have
been smooth as glass at times, and borderline scary at times.

Is "Protected Waters" a non-ocean body of water, with the added caveat of
"use your common sense regarding wave height?"