If you fly with a reasonably sophisticated GPS or Flight Management System, then you can usde the system to plan your vertical path during (usually) the descent. If you have to deviate from that plan - usually because of an ATC instruction, then you are off the planned path, called an "offpath" descent.. .. .On the Boeing FMS you define your vertical path with the various vertical points you whish to be at, and then select either PATH descent or SPEED descent. In the path mode the aricraft pitches up and down to follow the plan, and your speed will vary, according to the accuracy of the winds you have told the FMS to expect on the way down. In the speed mode, the aircraft will follow the planned descent speed, and you may drift off the path - the drift will be displayed on a path indicator in a glass cockpit.