Two questions that look like one
Almost every application asks whether you are legally authorized to work, and then asks whether you will need sponsorship. They are not the same question and they do not have opposite answers.
Read them side by side and the difference is obvious. Are you legally authorized to work in the United States? is about right now. Will you now or in the future require sponsorship for employment visa status? is about what the employer would have to do to keep you. Somebody working on a visa that runs out can answer yes to both, truthfully, and that combination is common enough that any employer reading applications has seen it many times.
The trouble starts when something treats them as one question with a yes side and a no side. Copy your answer from the first box into the second and you have inverted it. That is not a typo on a form; it is a statement about your immigration status made by accident, on a document you signed.
What this tool does with it
It holds them apart. An answer you gave to a question about authorization is never reused to answer a question about sponsorship, however similarly the two are worded, because the rule that decides whether two questions are the same question refuses any pair whose polarity is inverted.
If it cannot prove an answer from something you entered yourself, it leaves the box empty and flags it. You answer it once, on your own screen, and it's reused the next time an application asks it in close to the same words, unless reusing it would no longer be true. The application then stops at the employer's review page and waits, because you are the one who presses Submit.