The GI manual is shipped with all of our rifles.
The various mounting options are almost infinite, depending on the optic to be munted, where you want to put it, and hw you want to mount it.
The Standard A2carry handle would require a second mount, whether mounting on top, or goose-necking forward and down to place the optic over the handguard. The Tac Mt is primarily for red dot sights in a low ring so the sights show up in the lower portion of the optic. The Dominator is primarily for EOTechs,and was designed to put the dot out of perfect co-witness and elevate it to clear up the sight picture.
The various Aimpoinbt mounts normally go directly on to the A4 receiver, as do any of the EOTechs as their base is designed for flat-top co-witness.
Magnifying optics change things a bit, as you have to factor in eye relief, clearances for the bells of the scope, height of ring (no two companies that I know of use the same measurements for L/M/H), etc....
Hope that helps a little bit. As others have already pointed out, the Optics forum is a good source of info etc...