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 mahogany (.5) reflect combines red and violet in a rich, deep undertone. Less intense than pure red but more sophisticated. Ideal for adding depth without excessive warmth.
Each brand codes level and reflect differently. The table above shows how the major professional brands name this same shade: level 5, mahogany reflect.