In AutoCAD, what is the difference between natural and forced dimensions?

Personally, I have not encountered the words natural or forced when it comes to dimensions in AutoCAD. But if my suspicions are correct, you may be referring to the scaled and unscaled dimensions.

Scaled dimensions are dimensions that are generated from the reference system gauge of the AutoCAD engine that are calculated based on the default vector or raster datum, regardless of the unit that you may want to use. For example, while you are dragging the cursor when generating a line (L), a corresponding dimension near the cursor also appears and shows the length of the line. This is the scaled dimension.

Unscaled dimension is when you wish to override the scaled dimension generated by AutoCAD with some custom dimension of your own choosing or preference. This could appear forced because you would be forcing your own values to fit your needs because if you accept the default AutoCAD values, they may not fully fit your needs or your page.