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Event attribute proportional_depth that is getting added to all events contains a more reliable scroll depth metric than the traditional one.
Traditional scroll depth contains the percentage of the page that the user has already scrolled through. While this metric may be useful, the meaning of a 25% scroll depth may greatly vary between different screens and devices. The narrower the screen becomes the more the user has to scroll to get to the end of the page (or to 25%) so scroll depth is not really comparable between devices and screens as lots of articles detail this.
Proportional depth on the other hand is computed with the following simple formula:
proportional_depth = pixels a document has scrolled / height of your browser viewport.
Propertional depth basically tells how many browser windows fit into the amount of the page that has been scrolled through.
Why is it useful?
Proportional depth is comparable between devices. You can come to conclusions like on desktop users are only willing to scroll 3x as much as it is visible above the fold on page load, while on mobile it is 10x due to different usage behavior. You may then use this knowledge and organize site content in a fashion that most users see what is important on all devices even if it does not fit above the fold.