Basic Lighting Wiring

[mstrlucky74]

Basic dumb wiring question. So the second diagram adds aswitch leg. What the difference in wiring where it calls for one switch leg as opposed to two if it’s one circuit…12/3 instead of 12/2? How is the wiring to theselights usually done in the field? The feed is brought right to the switch box,I believe? So it's an extra wire(switch leg) between lights.

 

[John120/240]

The bottom diagram the first & third fixtures are switched by E, second & fourth fixtures switched by G. Why did they specify this way ? To give variable light levels. Or you could separate the lamps in a 4 lamp fixture. Two inside lamps switched by E, two outside lamps switched by G.
The extra wire E or G would just pass through the fixture when not used by that fixture.