This is my understanding of this stuff. Caster can be thought of as the angular measurement of the relationship between the upper and lower ball joints. Think of a chopper motorcycle at one end of the adjustment range (positive caster) with the wheel sticking way out in front of the steering stem and a fluttering shopping cart wheel (negative caster) wheel bent backwards from hitting too many objects.
If you could draw a line between the two ball joints, the line could be perpendicular to the ground (0 caster) or leaning backward like a chopper (positive caster) or forward like a shopping cart wheel (negative caster).
On Mopars, the upper control arm pivots are used to position the upper ball joint to set both camber and caster. However, by having and adjustable strut rod, the lower control arm can also be pulled forward or backward to add or reduce caster. Don't adjust it too far or you can induce binding of the upward and downward movement.
