Fire is the most common of all reactionals and covers effects that produce flames. It is possible to create heat by itself.
Magic
Natural fire requires three components: oxygen, heat, and fuel. As long as all three are there, the fire will continue to burn. Fire magic allows for the provision of all three components but at an increasing cost. It is cheaper to create heat of oxygen and fuel is provided whereas it is more expensive (and therefore produces a stronger resonance) when the magic needs to provide heat and fuel.
Crystal Sphere Techniques
In the Crystal Sphere Techniques, fire is considered the combination of fire and heat as a single sphere.
Towler Classification
In the Towler Classification, fire (“fragri”) is both a second order element (frati fagri) underneath reactionals and a third order which is the specific concept of a heatless flame (frati fagri fagri). The terms “second order fire” or “third order fire” is used to reference either magic that results in heat and fire (second order) or just flames (third order).