Air conditioning in your house is 50% correct airflow.
The manual J is an engineer or someone who knows the program telling you what tonnage your house requires based on different variables. The Amount of people(~500btu of heat per hour per person), type and amount of insulation, trees covering your roof, cubic feet, window type, window size, which way the sun hits your house, etc.
The manual M kinda finds out what the flex in your house is worth. Like total volume and length and stuff. Flex is the silver tubes in your attic/walls.
Finding out if your flex is too thin or full of wholes or undersized for your correct tonnage is just as important towards the performance of your AC as is the amount of zeroes you pay for the unit. It is just filthy, hot, miserable work. If it is undersized in return or supply it hurts your unit and your electric bill the entire time you have it.