What is a "prop" and an "accessory"?
Props / Accessories are meshes that attach to your sim. Some of the time these are meant to be part of the sim or an outfit despite being separate from the sims head or body mesh. Hats, glasses, elf ears, wings and the like are all examples of this sort and are usually called "accessory". Other times the prop is just meant to be used briefly, the apples or drinks taken from our Well Stocked Fridge, for example, and is then called a "prop".
As with normal head and body skins prop are made up of 3 files working together: A CMX, one (or more) SKN and a BMP.
Unlike objects there is no coding built into these items. They are simply skins attached to the Sim's body or head and so you'll find they are made up of the same 3 file types as a sim's skin: bmp, cmx and skn. Coding is used to attach and detach the temporary props. Skinning techniques (editing a skin's CMX file) is use to attach permanent ones.
How and when is a prop used?
Props/accessories cannot exist in the game without attaching to a sim. They cannot be placed on the floor or a surface and cannot be attached to a furniture object because the only objects that use skins are Sims. Because of this some clever object/prop swapping is needed in some objects.
A coffee maker is a great example. When sitting unused on a counter the coffee maker's carafe is part of the object. When a sim reaches over and "takes" the coffee pot what is really happening is that the graphic on the object changes to one that has the pot missing and, at the same time, a prop that looks like the coffee pot is added to the sim's hand.
This is also what happens when they answer a phone, pick up the basketball, ride a mechanical bull, play a guitar or put on a VR Helmet. This is the reason an item can suddenly look different when a sim is holding it in their hands - it IS different.
When recoloring a prop or accessory ...
Sims' heads and bodies have the ability to apply many different textures (BMPs) to the meshes allowing you to install a mesh only once, no matter how many recolored outfits you have installed for that mesh.
Props do not work this way. Props and accessories have specific textures (BMPs) assigned to their mesh. The only way to recolor a prop is to make a new copy of the mesh for each color. This is done by opening the copied SKN file in a text editing program and changing the second line. On this line you list the name of the BMP that this SKN should use. Only one BMP may be listed and you do not include the file extension.
To keep the files mac-friendly be sure to keep your file names shorter than 31 characters (27 for the name plus 4 for the period and file extension)
Who uses these files?
Object Makers and Skinners both have reasons to use accessory files. Skinners usually make the accessory a permanent part of the body or head. Object makers usually use hem as props, an item that is added to the the sim by an object. While most object makers use the props for specialize instances (while playing guitar, or reading a book) some sites such as Odd Sims and Some Simthings have made "prop adders" that allow you to attach a prop (a hat, a sword, sunglasses) to a sim indefinitely, allowing you to de
The player does not have any special need for these files. For the player these are just accessory files that come with a downloaded skin or object.