AZW? and MOBI# Format

From what I can find, it appears that AZW3 is MOBI 8, but I've seen different references to AZW being MOBI6 and MOBI7.

Can anyone tell me how the AZW? format relates to the MOBI# format?

Also, what is the difference in the MOBI Header between MOBI6 vs MOBI7 and MOBI7 vs MOBI8?

Links to resources would be great if they exist.

Thanks,

Eric