What's the point of a sd atlas?

[abcd]

In Unity, I tried to create two buttons using hd and sd atlas, they look identical. So does hd/sd switching only necessary in PixelPerfect Mode? While in FixedSize Mode I only need to use the highest resolution atlas available, NGUI will scale ui elements based on screen resolution?

[ArenMook]

Fixed size means nothing is going to be scaled -- precisely how it's meant to work. Pixel perfect means the UI will scale depending on the screen's height. When working with HD/SD atlases, keep the UI root in fixed size.