I'm a bit unclear as to what the widths achieve in data-srcset

[AdamDSherman]

Eg with

<img
    data-sizes="auto"
    data-src="image3.jpg"
    data-srcset="image3.jpg 600w,
        image1.jpg 220w,
        image2.jpg 300w,
        image3.jpg 600w,
        image4.jpg 900w"
    class="lazyload" />

Are these the constrained max proportions for each image?

Or are these the breakpoints at which the images are used?

Is it the same for background image srcsets?

Many thanks