Is this only for the private pilot rating or does it apply to the
commercial pilot rating as well?

As an aside question, has anyone used exams4pilots.org to prepare for
the commercial glider pilot rating? I have gone through several
practice tests and noticed that several questions are given that have
nothing to do with glider flying. As a commercial glider pilot why do
I care about fuel burn, manifold pressure and how the engine works? It
is a convenient free website, but as it relates to preparing for the
commercial glider pilot exam, the website leaves a bit to be desired.