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.
You can add CMC to any type of sugar paste:
  • fondant (sugarpaste)
  • gum paste (flower paste)
  • mix of fondant and gum paste
  • modelling paste
CMC is usually added to fondant or to a mix of fondant and gum paste. Those two types of pastes are too soft to make cake toppers. If you don't add CMC to them, the lower parts will sink under the weight of the upper parts.

Plain gum paste and modelling paste hold the shape well, so there's no need to add any CMC to them. But not all brands are the same - some are quite soft and don't dry out quickly. If you want them to be thicker/firmer and you want them to dry faster, you can add CMC to them.