Some Simple Questions about Lightmaps

I had a few questions about lightmass and the lightmaps it generates, any help even just pointing me in the right direction would be helpful, thanks in advance!

  1. What is the difference between HQ and LQ lightmaps, when does each get used?

  2. What specific information is packed in the R,G,B,A of a lightmap texture?

  3. What is the difference between a Lightmap and a SkyOcclusion that is generated, any special considerations here?