The customer sent me a request to update their Desktop Wallpaper, with the comment

Don’t use fit as it upsets the “look” of the wallpaper.

We mostly use “Fill”, so the customer should be ok.  It got me thinking through.

How do all the “Choose a fit” settings behave?

Fill
The image is centered on the screen. The image fills the screen horizontally and vertically, and maintains its original aspect ratio. Parts of the image might overrun the left and right sides or the top and bottom edges (but not both).


Fit

The image is centered on the screen. The image fills the screen horizontally or vertically, and maintains its original aspect ratio. Parts of the image might not fill the left and right sides or the top and bottom edges.


Stretch

The image is centered on the screen. The image fills the screen horizontally and vertically, but does not maintain its original aspect ratio. No part of the image overruns the screen.


Tile

The image is anchored in the upper-left corner of the screen at its original size, followed by as many copies as are necessary to fill the screen. Parts of the right-most and bottom tiles might overrun the edges of the screen.


Center

The image is centered on the screen at its original size.

Span
When you have multiple monitors connected to the computer, this option stretches the image across the monitors.
Microsoft Press – Personalize Your Working Environment in Windows 10