The sphere that represents the main focus of an effect is called the primary sphere. For example, a ball of fire’s primary sphere is Pyrotechnics, whereas an effect that picks up a boulder and hurls it has Kinetics as its primary sphere. Other spheres are often mixed in, but the primary sphere will always be the one that represents the basic function behind the effect.
As a rule, an effect has only one primary sphere. Magic complex enough to require what might seem like multiple primary spheres takes longer, and requires the Mystic Charge feat to pull off gracefully. Such combinations usually produce multiple effects that occur in some specified order. For example, an effect that uses Displacement to teleport a dagger behind an enemy and Kinetics to thrust the dagger into his back could not be performed as a single action, but the Displacement could be charged, then released the following round as a free action immediately before a standard Kinetics effect.