Question on FAA knowledge test endorsements.

Simple question (for everyone but me!):

Is the CFI endorsement to take a private pilot test for a specific category
(e.g. airplane, rotorcraft, glider) or a category and class (e.g. airplane,
single engine land) or some other classsification (e.g. fixed-wing), or not
category or class specific?

I ask because I thought I would take some winter down time to work on
passing the knowledge test needed to earn a private pilot, glider
certificate. I do not hold any previous certificates (other than student
pilot.)

I was training at a glider club this summer and ground training was not
something they normally did - the local community college offers ground
school courses a couple times a year - but the next class doesn't start
till spring. I asked what other options they knew about and they thought
courses offered by King and Sporty's might be a possibility. That sounded
plausible but their DVD courses looked airplane-centric, so I contacted
Sporty's. I asked them if I passed their Private Pilot DVD course and got
an endorsement from them, would that endorsement allow me to take the
knowledge test for a private pilot, glider rating? Their answer was:

"The Private Pilot on line course endorsement is for the private pilot
fixed wing test.

Glider information is not covered in the course and the endorsement will
not include glider."

The "fixed wing" part kinda threw me - and now I'm even more confused. I
had no idea the tests had yet another classification - one in which gliders
aren't fixed wing aircraft! Geez - I hope there aren't any questions on the
FAA knowledge tests about the FAA knowledge tests - at this point I'd
flunk! :-)