The difference between motor max and batt max has to do with duty cycle. Duty is basically the decimal of top no load speed your motor is currently running at. Its also the ratio of amps in the motor to amps coming from the battery. Eg If the VESC is pulling 10A from the battery, and the motor is spinning at 0.5 duty the motor will be at 20A which is how you can have 2 limits at the same time.
Since RPM = V * kV, if you lower RPM then you need to lower voltage as well since kV is constant. Because of this, the voltage in the motor is not constant, while the voltage coming out of the battery is. The amperage on either side needs to be different for the same amount of power to be moving through the system on either side. Duty just represents that difference.
Duty cycle is lowest at low speed, so the ratio of motor amps to battery amps is high. Say you were at 0.05 duty and you were pulling 10A out of the battery, you’d have 200A in the motor, that’s enough to knock you on your ass, so you’d set a motor max limit to something smaller, usually people set it around 40-100A. The same concepts apply to breaking.