So in ENT and TOS "impulse" is some sort of fusion power - possibly even some form of pulse fusion like in the proposed Daedalus starship design. (I like how on book called it "InterMittent Pulse fusion"...or "I.M. Pulse".)
The book "Final Frontier", featuring Capt. April and George Kirk in the early days of the Enterprise, was the first place I saw "Internally Metered Pulse Drive". Always thought that was a neat idea.
 hmm . . . Memory Alpha says that the Galaxy class arboretum was on deck 17 which would place it on the lower portion of the saucer
I believe Rick Sternbach indicated that those are
the arboretum windows, but the labels didn't make it onto the 1701D blueprints.