CMC stands for carboxymethyl cellulose. It's a powder that makes the sugar paste thicker/firmer (the formed parts hold the shape better) and speeds-up the drying time. Other names for CMC are tylose powder and tylo powder.

CMC is NOT the same thing as gum tragacanth. CMC is a synthetic substitute for gum tragacanth. CMC works quicker than gum tragacanth, so you don't have to wait many hours for the paste to get thicker/firmer.
The amount you need to use depends on:
  • The type and brand of paste you are using (the softer the paste, the more you need to add) .
  • The part you want to make:
    • parts that support the weight of other parts should have more CMC
    • parts that don't support any weight don't need that much CMC. 
  • The amount of modelling involved:
    • more complex shapes require more time to model, so the paste should be a little softer
    • simple shapes f.ex. a flattened oval, don't require a lot of time to model, so you can add more CMC. 
  • Your preferences:
    • some cake decorators prefer to work with softer paste and let it dry for longer
    • some cake decorators prefer thicker paste that dries out faster.