App icon in UWP

The MSDN documentation provides clear description of where each of the images is used. In the bottom you can see Asset size tables tables which contain all the dimensions and their usage.

As for the Square 44x44 logo, the first five images in the first row are used for different scales in the app list.

The target images are used for app icons in different places around the system. In their unplated form, they should have transparent background. You can provide just the sizes recommended by the form, but you can also provide more sizes (see the documentation for the complete table).