If you only specify a single ui_resolution of "hd" or "fhd" in the manifest, it will scale automatically between the two. Specifying both "hd,fhd" tells the renderer that your layout/code includes images and coordinates for both. For SD, there is no auto-scaling.
There's built-in support for specifying different resolution images via the uri_resolution_autosub manifest setting, but I haven't yet figured out the best way to deal with multiple resolution coordinates aside from explicitly modifying all of the applicable locations and sizes in code, which is a pain.
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