There's a question in the FAA commercial question bank that indicates that squall lines are nonfrontal, and the reference for the question points to the Aviation Weather Advisory Circular.
The word, "nonfrontal" (or any variation of it ie "non-frontal") does not appear in that document. Also, the word "squall" appears four times, and none of those seem to have information that indicates that they are nonfrontal.
Where is the FAA providing this information? Is it actually correct?