Microstep Mode Switching or just Mode Switching: Using AXISn_MICROSTEPS to set the micro-step mode used during tracking and AXISn_MICROSTEPS_GOTO to set the micro-step mode during gotos. Configuring both of these (as opposed to just AXISn_MICROSTEPS) allows potentially higher RPM's by reducing the required step frequency to within what the micro-controller is capable of (fewer step pulses for each rotation of the stepper motor shaft.)
MaxRate: Sets the the maximum goto mode pulse rate (and default goto speed,) in microseconds per micro-step. Lower numbers are faster. The motors and drive design will limit this. Each particular microcontroller also has a base "minimum" that this can be set to. Note: MaxRate is in units of microseconds per AXIS1_MICROSTEPS mode micro-steps.
Using micro-step mode switching allows for a lower minimum MaxRate. For example, AXISn_MICROSTEPS 16 and AXISn_MICROSTEPS_GOTO 2 takes 8 tracking mode sized micro-steps for each goto mode micro-step. So the MaxRate is allowed to be almost8x lower. We say almost since the microcontroller might need to do a little more work to mode switch and if so can't step the motors quite as fast so you end up "only" being 4x or 6x faster.
Also, MaxRate is the default rate. Inside OnStep there are controls to adjust the goto speed to be double or half the default speed. It's a good idea to account for this in your MaxRate setting so once you determine the fastest speed (lowest default MaxRate) double it to set the default. Then in OnStep you can adjust goto speed from the fastest rate to one quarter of it.
When compiling OnStep there will be warnings and/or errors if your settings aren't reasonable. These checks are not fool-proof since different steps per degree settings for Axis1 vs. Axis2 can affect the allowed MaxRate but it usually comes close.
Steps Per Degree: When tracking, the number of steps a motor axis takes to cover one degree of the sky.
StepStick: A widely used format in the 3D printer community for Stepper Motor drivers. They are small rectangular boards, with 8 pins on each side of the board. A variety of models are available with different current ratings and features.