See the following post from a user which actually provide a good description of these Render Modes.
SDF8, SDF16 and SDF32 are over sampled modes (requested sampling X over sample) where the SDF is computed on a binary representation of each glyphs. This mode is extremely slow but very accurate.
SDFAA and SDFAA Hinted are modes where the SDF is computed on the sampling point size using an 8 bit alpha (ie. anti aliased) representation of the glyphs. This mode is fast enough for runtime use but less accurate. The less accurate part is not visible on glyphs that are smaller than 72 to 96 point size on screen displayed at 1:1.