NINE_SLICES - https://docs.unity3d.com/Manual/UIE-USS-SupportedProperties.html (Slicing section)
https://forum.unity.com/threads/how-does-slicing-in-ui-tookkit-works.1235863/
https://docs.unity3d.com/Manual/9SliceSprites.html
https://developer.mozilla.org/en-US/docs/Web/CSS/border-image-slice
CENTER - CENTER enables the texture to be rendered centered in relation to the
element. If the element is smaller than the texture then the background
should use the element as stencil to cut off the out-of-bounds area
STRETCH - STRETCH enables the texture to cover all the area of the container,
adopting its aspect ratio.
NINE_SLICES - https://docs.unity3d.com/Manual/UIE-USS-SupportedProperties.html (Slicing section)
https://forum.unity.com/threads/how-does-slicing-in-ui-tookkit-works.1235863/
https://docs.unity3d.com/Manual/9SliceSprites.html
https://developer.mozilla.org/en-US/docs/Web/CSS/border-image-slice
CENTER - CENTER enables the texture to be rendered centered in relation to the
element. If the element is smaller than the texture then the background
should use the element as stencil to cut off the out-of-bounds area
STRETCH - STRETCH enables the texture to cover all the area of the container,
adopting its aspect ratio.