The background position CSS attribute tells where the top left pixel of the image is going to be relative to the top right pixel of the container. In addition to pixels, there also keywords and percantages, which work a bit differently.
The values are in order HORIZONTAL VERTICAL.
So in this particular case 100% means align the right edge of image with the right edge of the container.
The next value means pull the top edge of the image up 216px
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