This is the most popular mode for assisted flying in Plane, and is the best mode for inexperienced flyers. In this mode Plane will hold the roll and pitch specified by the control sticks. So if you hold the aileron stick hard right then the plane will hold its pitch level and will bank right by the angle specified in the ROLL_LIMIT_DEG parameter (in degrees). It is not possible to roll the plane past the roll limit specified in ROLL_LIMIT_DEG, and it is not possible to pitch the plane beyond the PTCH_LIM_MIN_DEG or PTCH_LIM_MAX_DEG settings.

Note that holding level pitch does not mean the plane will hold altitude. How much altitude a plane gains or loses at a particular pitch depends on its airspeed, which is primarily controlled by throttle. So to gain altitude you should raise the throttle, and to lose altitude you should lower the throttle. If you want Plane to take care of holding altitude then you should look at the FlyByWireB mode.

In FBWA mode throttle is manually controlled, but is constrained by the THR_MIN and THR_MAX settings.

In FBWA mode the rudder is under both manual control, plus whatever rudder mixing for roll you have configured. Thus you can use the rudder for ground steering, and still have it used for automatically coordinating turns.