@Liz, the descriptors are part of the srcset syntax, both w and x descriptors each have their own uses. They are not used together, it’s either one or the other. w-descriptors provide the browser information on the width of the image, while x-descriptors provide the browser information on the pixel density of the image.
When I mentioned the srcset attribute alone being sufficient, I was referring to usage of the element. requires srcset to be present, but srcset can work alone without picture.
Hope this clarifies things a bit.