Gender is identified with 'f' (female) or 'm' (male), while age is identified with 'a' (adult) or 'c' (child). The next two letters identify gender and age, respectively. Maxis always used numbers, but creators could, and often did, use letters. The next three characters are an identifier for the mesh. Formal, Swimwear, Winterwear and High Fashion skins are identified by their initial letters, though players should note that hand files also begin with 'H'. Pajamas are identified with 'L' (for Lingerie). Everyday skins are identified with 'B' (for Body).

Heads are identified with 'C' (for Cranium). The first letter identifies the type of skin. In The Sims, all information about a skin file is carried in its filename. Likewise, buyable pajamas, swimwear, etc., for children with custom body types must be labeled as being for the 'chd' body type.

Therefore, buyable pajamas, swimwear, etc., for adult Sims with custom body types must be labeled as being for the 'fit' body type, or those Sims will not be able to access it. However, if Hot Date or later is installed, clothing racks will see adult Sims with custom body types as 'fit'. Custom body types are possible, and they can have their own three-letter identifiers. The basic body types are 'fit', 'fat', 'skn' (skinny), and 'chd' (child). (a censor blur appears on top of a naked Sim). There are also nude skins for when a Sim is using a toilet, using the Vibrate interaction on another Sim or is taking a bath or shower. Skins are available for different modes of dressing: Everyday, Pajamas, Swimwear and Formal. Some creators made 'pale' versions of the light skin tone and made them available for use by others. True custom skin tones aren't possible, but variations of the basic skin tones are, as long as they're named so that the game engine 'sees' them as light, medium, or dark tones.

However, if a Sim appears to be wearing accessories such as glasses, it is usually because they are painted onto the Sim's head or body skin. Some skins have accessories such as hats and glasses, which have separate BMP and mesh files. For example, a skin might have white hands so a Sim using it will appear to be wearing white gloves. A Sim's hands normally match the Sim's skin tone, though it is possible for a skin to have custom hands. Originally, the default formal outfits used formal hands, but these hands were not used after Hot Date. However, a Sim's hands are separate hands are not part of body skins, but are defined separately in the Sim's character file. Each image file is that Sim's entire body or head, including skin (there are 3 skin tones - light, medium and dark - and it is common for the same outfit to be made in all 3 tones) and hair. Both are simple 256 color (8-bit) bitmap images, numbered to correspond with the mesh file which gives them the desired shape on the Sim. Skins in The Sims are comprised of two parts per Sim: a head and a body.

Sims 3 realistic skin tone