A nine-slice atlas provider: the picture is a 3 x 3 grid cut out of the named texture. The four corners keep their size, the four sides stretch along one axis, and the centre stretches along both. It is how a frame or a panel is drawn at any size from one small sprite.
Slicing and the scaling of the edge sizes are described on IUiTextureDataProvider.
Vec2 = [0.25, 0.25] Width of the left and of the right column on screen, in pixels of the icon's layout.
They are independent of the texture: TextureUs chooses which pixels a column shows,
this chooses how wide it is drawn.
Most shipped sprites use the column's own pixel width, which draws the edge at its authored size when the screen matches the layout's resolution.
Vec4 = [0, 0.25, 0.75, 1] The four column boundaries of the grid, left to right, in pixels of the source
resolution: the left edge of the sprite, the two cuts, and the right edge. Each is
divided by mTextureSourceResolutionWidth.
Vec4 = [0, 0.25, 0.75, 1] The four row boundaries of the grid, top to bottom, in pixels of the source
resolution. Each is divided by mTextureSourceResolutionHeight.
Vec2 = [0.25, 0.25] Height of the top and of the bottom row on screen, in pixels of the icon's layout.