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Receptor exposure vs contrast
mAs sets how many photons reach the receptor; kVp sets penetration and subject contrast.
The rule
Receptor exposure is the amount of radiation striking the image receptor. Film texts called its visible result density. In digital radiography, mAs is the primary controller of receptor exposure: double the mAs and twice as many photons reach the detector. But the computer rescales brightness during processing, so a wrong mAs does not show up as a dark or light image. Too little shows up as noise. Too much shows up as unneeded patient dose and, at the extreme, saturation. The exposure indicator is how you see it.