Well, the 1-2-3-MAX scale simply indicates what kind of brake pressure you'd want when the aircraft's main gear touches the ground with the thrust levers at idle.
The MAX option is very rarely used since the aicraft decel will be VERY rough. Use this on very short and wet runways only  
The RTO stands for Rejected Takeoff. During takeoff, if you retard the thrust levers to idle at speeds of 90kts (85kts for the B767 I think)) or above, the aircraft will automaticly apply maximum braking. Normal brake condition pressure is 3000psi, selecting the MAX option will give you this. But you can get even more pressure with manual braking or with the RTO.

Those autobrakes scale varies with aircraft type. Airbus has a MIN-MED-MAX config, where MAX also functions as the RTO mode