I guess it will be all clear when in the sim or the actual airplane. Think of it as using the arming button and not. When flying manually, you use the arming button, when in autopilot, you don't. Instead you move the what Learjet calls the "barrel". If on autopilot, and a change in altitude is desired, move the barrel up or down (a few clicks, the more you push or click the bigger the movement of the control surface), just remember to set altitude preselect.
If flying manual, you push the arming button (aka control wheel trim switch) and move up or down in order to trim the stab.