From X-plane Wiki

The Limits screen allows the definition of instrument ranges (by setting a lower and an upper limit). This is not to be confused with the Limits 1 and Limits 2 tabs under the Engine Specs section of PlaneMaker, which actually defines performance limits, not just indicator markings.

The two basic instrument types covered are round and digital instruments.

Note:

• You can only define new limits for instruments listed on the limits screen - not for all that can be selected on the panel screen.
• You have to figure out the acronyms for the instruments yourself, Plane Maker offers no help.
• Some instrument variants have bitmap faces that won't reflect the new ranges - but the hands will! See Custom Instruments for a solution.

[edit] Round

1. Tick the checkbox to enter custom limit for an instrument
2. Enter figure: Minimum
3. Enter figure: Maximum
4. Enter figure: Angle of the hand for the minimum
5. Enter figure: Angle of the hand for the maximum
6. Tick the checkbox mirror if (ToDo: What does it actually do?)
7. Tick the checkbox halve over to get a scale with two different graduations. You can enter the switch-over figure in the numerical field that opens once the checkbox is ticked. See below for example
8. Tick the checkbox label if (ToDo: What does it actually do?)

[edit] Example for Halve Over

Typical application: "Condense" the high speed range of an airspeed indicator to leave more space for the low speed range, making it easier to read if you're close to the stall speed.

[edit] Digital

(ToDo: Describe digital ... I never use it)