A rune is a category of artifact that uses the medium that the rune is engraved (or written) on acting as the source of energy and have limited effects.
Surface Area
Runes are an effect written in on two-dimensions. More complex effects require exponential surface area which limits most runes to the size of the material which then limits the effect of the rune. Even controls, such as turning it off and on, increase this area which is why the Farimon chamber and fire cores are built on the idea of an “always on” fire rune.
Energy Source
Runes can be written on almost any surface which makes them relatively efficient to create. Destructible or flexible materials, such as paper or cloth, may only provide a small amount of energy before they are expended (usually destructively) but runes can also be engraved onto harder surfaces such as metal or crystal which provide a more stable supply of power.