Some calipers (a lot) have different sized pistons. They are designed for the small piston to make contact first then the large.
Confusing cos the disc is a circle, imagine putting a dot on the disc and rolling the car forwards. The dot should go past the small piston on the caliper then the larger piston. If the other way around crazy vibrations may occur and probably strange pad wear.
And the reason for the difference with the bleed nipple needing changing or being upside down is some cars have the calipers in front of the axle, some behind.
Sounds heaps confusing now that I have written it. Hope someone can explain it better!