Level 5 corresponds to a light brown on the universal colorimetry scale (1 = black, 10 = lightest blonde). It measures the lightness or darkness of hair, independent of the reflect.
The natural (.0) reflect indicates no added undertone. It is the pure base without ash, golden, copper or any other reflect. Used as a mixing base, for natural-tone gray coverage, or when a result without visible reflects is desired.
Each brand codes level and reflect differently. The table above shows how the major professional brands name this same shade: level 5, natural reflect.