Mana Conversion Affinity (魔力変換資質 Maryoku Henkan Shishitsu) is an innate ability of some mages to naturally convert their mana directly into another form of physical energy. What normally requires regulation via magic, these ability allows to occur spontaneously. It becomes simple to convert one's own mana into energy, and when doing so intentionally, this can even occur at high efficiency.
Only two types of MCA have been seen so far: flame (炎 hono'o, also 炎熱 en'netsu, i.e. thermal energy) and electricity (電気 denki, usually misnamed "lightning", i.e. electric energy). Magical Girl Lyrical Nanoha A's Visual Fan Book, however, lists freezing (凍結 tōketsu) as a rare type of MCA, although no mage possessing such an ability is depicted in the series, making its canonicity unclear. In addition, despite a number of spells based on other elements (e.g. Shamal's wind-based spells, Corona Timil's earth-based spells), no other association with MCA has ever been revealed.
Normally, an individual with such ability can only possess one type of the above.
In the Brave Duel continuity, a similar game setting is implemented as (elemental) specialization, which includes all the types of MCA above as well as other non-MCA elements.