Okay. In basic terms, the procedurally generated (procgen) sectors in the galaxy are big cubes of 1280 ly x 1280 ly x 1280 ly. They have names like WREGOE and STUEMEAE.
When you see a system name in a procgen sector, it will be in the format something like: STUEMEAE EG-Y
D37 or OUTOTZ UJ-Z
B46-3
The bold letters in the examples there we call the "mass code" and they are a general indication as to the size of the primary star you will find in the system. They run from H (biggest, most massive stars - usually) down to A (smallest, least massive stars).
If you think of the whole sector as being a big cube, then there is only one set of mass code H stars in it. That is divided into 8 smaller cubes (called subsectors, or boxels) of mass code G stars. And divided into 64 smaller cubes of mass code F. And so on down to mass code A and B. There are lots and lots of those.
(edited to add: )
When Chiggy says "search only A8, A9, and F0-F4" though, he doesn't mean the mass codes - he means after you have gone to a mass code D subsector (like STUEMEAE AA-A D) and then start looking for stars whose
spectral class (which you can see on the galmap) is A8, or A9, or F0 through F4. In practice, they will be something like F4VB, but you're looking at the first two letters of the spectral class.
*grinning* Isn't it fun!
