Difference between the two rules of react hooks

I was reading through their docs and found the below

Rules of hooks:

✅ Call them at the top level in the body of a function component.
✅ Call them at the top level in the body of a custom Hook.
function Counter() {
  // ✅ Good: top-level in a function component
  const [count, setCount] = useState(0);
  // ...
}

function useWindowWidth() {
  // ✅ Good: top-level in a custom Hook
  const [width, setWidth] = useState(window.innerWidth);
  // ...
}

My question is, what is the difference between the first and the second rule. The first rule itself covers what the second rule is telling.