Code:
{title}.azw3
{title}.sdr/
{title}.sdr/{title}.azw3r
{title}.sdr/{title}.azw3f
{title}.sdr/StartActions.data.{asin}.asc
{title}.sdr/EndActions.data.{asin}.asc
The *.azw3 file is your book. Everything in the *.sdr folders are annotations data.
Note: Enhanced typesetting books (KFX?) appear from preliminary reports to violate this rule. But they also seem to be a new format.
We'll see how things go, as far as that goes.