Re: Meaning of "within"

From the preface right before the "Key to Soil Orders" is the closest thing to a definition of "within" that I can find.
It seems somewhat circular to me.

"If diagnostic horizons or characteristics are criteria that must be 'within' a specified depth measured from the soil surface, then the upper boundary of the first subhorizon meeting the requirements for the diagnostic horizon or characteristic must be within the specified depth"