A Charm is an extraordinary feat you can perform, stemming from supernatural mastery of your skills. Many of the Charms in Exalted are appropriate Charms for Radiant, but not all—for a Charm in this game to work, it needs to have a specific descriptive effect. That is to say, it cannot work invisibly. Charms have two functions: they can enhance your stunt bonuses for relevant actions, and they can create significant descriptive effects. Charms enhance your actions. They are not firmly tied to specific Abilities, but they may be loosely associated: I believe you can find a way to use Thrashing Carp Serenade with Medicine, but likewise I believe it would be a challenge to engineer that situation.
You can use a Charm unresisted to create a self-involved effect, such as leaping to your feet with Effortlessly Rising Flame or summoning a weapon with Glorious Solar Sabre. Affecting other characters, such as if you should attempt to amputate a liver with Convulsive Displacement Infectious Atemi or seduce an innocent with Hot-Eyed Snake Whispering, permits their passive or competitive resistance. It will usually be evident when a Charm usage is self-involved; the ST will determine whether an aggressive Charm use is resisted passively or competitively.
Charms that affect the environment should be applied against a static obstacle. For instance, Harvest of the Hunter could create a crop of arrows sufficient to supply an entire squadron, if it is invoked with sufficient élan. Feel free to use the Charms in Exalted or devise your own as the spirit moves you.
Regard Relics of the Ancient World and Extraordinary Possessions as mystical objects that can be used like Charms; for instance Reborn Glacial Rain might be a dagger that freezes wounds with its icy bite, and reforms itself from the waters whenever it is destroyed.
Regard these as free-willed entities that are often present and sometimes absent, loyal to and willing to come to the aid of the character; their assistance can be used like a Charm. They may exact a price for larger favors.