If I create a light and set its range to 8, what does that "8" mean exactly? 8 units?
Exactly as you said, it means 8 units. What the actual units are is entirely up to you. If the game is based on space, perhaps a unit could be a light year. If your game is based on insects then perhaps a unit could be 1 centimeter or for that matter a unit could be 39,563 decameters if you want it to. It really doesn't matter as long as everything else is in proportion.