What is Surface Flatness? What is the difference between local & global flatness while using the Surface Flatness Tool?

Flatness is a common symbol that references how flat a surface is regardless of any other datums or features. Flatness is a 3D tolerance zone that only controls the form of a feature. The tolerance zone includes two sets of parallel planes where the entire referenced surface must lie. It can be gauged only if the surface is held perfectly parallel to the measurement device. Source: https://www.gdandtbasics.com/flatness 

The Gocator Classic interface offers the Surface Flatness tool which allows you to calculate local flatness of several regions (up to 15) and also returns global flatness. If you're using only one region, the Global Flatness and Local Flatness will be the same.

Global Max - Global Min = Global Flatness

Local Max - Local Min = Local Flatness

For more information on the surface flatness tool - Link to manual

It is to be noted that Surface Flatness is different from Surface Roughness. Surface Roughness is a subset of Surface texture which includes Form, Roughness and Waviness.