Adjusting the lower suspension mounts on an RC car lets you alter the roll center, track, toe/sweep, and anti-squat/kick-up (depending on which end of the car it is). This set of options is fairly complex already:
Often those adjustments are made by changing the "pill inserts" that hold the front and rear of the pin that holds the lower suspension arm.
However, when we look at the upper links, sometimes the situation gets weird.
Most cars have simple upper links that be be moved up or down (to adjust roll center) or in and out (to adjust camber gain.) However, some cars now take this a step further - they have upper arms that use the same pill insert system from the lower arms, allowing extra degrees of adjustability.
With the upper arm's pill inserts, you can adjust:
So caster gain is the new setting we've gained using the inserts. (You could in theory also move the front of the arm in/out and the rear of the arm the opposite way, but that'll get super weird. Don't do it.
In general we recommend leaving caster gain alone and just moving the inserts so they are equal (e.g. if you go 1-out on the front, do the same in the rear.) Caster gain is for advanced tuning - once everything else on your car is working well, then try playing with caster gain to see how it affects your car's handling.
Please keep in mind, when moving the inner end of upper arm in or out, it will change your camber unless you adjust the outer end of the link to compensate, either by shortening the upper link or by moving the outer link mount.