How far can a character see? Can they see in the dark? How good is their sense of smell? These sorts of questions about the character and the environment are highly complicated. To answer them exactly would require precise biological details of characters, many of whom are of fictional species. It also requires an exact understanding of the environment, its temperature, light levels and layout.
Rather than tackle this impossible task by defining arbitrary values for each type of creature, the 6d6 RPG uses common sense. A character is always assumed to be using their abilities, the environment, the situation and equipment (mundane or otherwise) to the full.
A human exploring a dark cave is assumed to have a torch or appropriate light source. An alien from a planet with a methane-based atmosphere will have breathing equipment when they visit Earth. Through their equipment and abilities, all characters and creatures are placed on a level playing field with regards to the environment.
Boban is creeping with a flickering torch through the cellars of an abandoned house but is unaware of the giant rat that is stalking her though the darkness. Neither Boban nor the rat have any advantages over the other in this situation. The rat is a creature of the dark but Boban is using a torch. Both characters are equally advantaged and disadvantaged by the situation.
To gain an advantage, a character (or creature) must have a card in their deck. One of the most powerful of these is a card with a Environment keyword such as Dim Light. These cards are usable in almost any sort of action that a character performs in a particular environment.
Boban stops and takes a look around. Using a pseudo-card for awareness with her Dim Light card, she has 2d6+0 awareness check. The rat does not have any specific environment cards or stealth cards; instead it relies on its Diminutive card for a 1d6+3. The rat scores five whereas Boban gets eight. At the edge of her torch light, Boban spots the rat.
Boban draws her dagger and throws it at the rat. Using her Dagger and Throw Cards Boban has 2d6+2 but by including the Dim Light card as well, she can manage a 3d6+2 attack. Things are looking bad for the rat.
To successfully notice a hidden creature or object, the awareness action must beat (score more than) the resistance to be successful.