Human Interface Guidelines image size/resolution

Hello,

I have a question about something mentioned in the Human Interface Guidelines in the section on image size and resolution ( https://developer.apple.com/design/human-interface-guidelines/ios/icons-and-images/image-size-and-resolution/ ).

It says to use an 8x8 pixel grid when creating images. What does it mean by this exactly - to ensure that the width and height of the image are multiples of 8, or to ensure that all image details have a size of at least 8x8?

What kind of image could I create to see the effect of not doing it right? (I have placeholder artwork in my app, which is not 8x8 in either sense, and I don't notice it looking any different on different resolution iOS simulators. The placeholder artwork isn't meant to be very good anyway though).