Please note, this is not a costless online course !?
Which setting do you mean? (what product you are talk about [please add this information to each of your threads, thx.])
AutoCAD works unitless, but some settings and feature have some kind if units
For modelspace in AutoCAD and AutoCAD LT, it is the variable INSUNITS [F1]
For new files based on templates (*.dwt), set up the template to the setting you want.
For new files NOT based on templates, MEASUREINIT controles the set up
for imperial or metric usage, insunits default values are 1 for imperial(inch?) und 4 for metric(millimeters).