The guys in yellow/black stripes move the chains. The guys with a big red X on their chest carry extra footballs. What do the guys with the big red K on their chest do? I can’t believe I don’t know the answer to this after watching football for 30 years.
The guys with the K on the bibs carry the balls that are specially reserved for kicking situations.
In the past, teams would bring out older balls for kicking situations because they would travel farther because they weren’t as slick and they had expanded a bit.
Now the NFL requires that pristine balls be used in kicking situations only.
I don’t think it’s made much of a difference as kickers are still making long field goals and punting averages have held steady.