By default, the challenge coins are 2D coins if without further notice.
If consider enamel pin as is 1.0 then challenge coin should be 2.0. Challenge coin is sort of the upgrade version of enamel pin: Most of pins are one side pin, while most of coins are two sides coin. We classify challenge coin as : one side (color fills) coin, two sides (color fills) coin, no color coin, 3D coin, two sides 3D coin.
One side (color fills) coins means color only fills on one side of the coin. While there will be design/ pattern on both sides, but only one side is colored.