Sutra:
66. makAram irupaththanjAm aksharamAy njAnavAsiyumAy irukkaiyAlE AthmAvais sollukiRathu.

Meaning:
The makAram that follows the ukAram is the 25th letter (starting from ka); it also describes knowledge as it arises out of the root for the word gnAna. Therefore,it stands for the Atma which is the 25th tattva and whose nature and quality is gnAna.

Sri PBA Swami's Sarartha Deepikai:
The third part of praNavam which is makAram is now explained.

It is said that each of the aksharas explains one tattva. The 24 aksharas starting from ka till bha explain 24 tattvas and the 25th akshara which is ma shows the jivAtma which is the 25th tattva, as in "pongkaimpulanum poRiyainthum kaurumEnthiriyam aimbootham, ingivvuyirEy pirakiruthi mAnAngkAra manangkaLE".

Also, makAram arises out of the root "mana-jnAnE" or "manu-avabodhane" and as such it stands for the Atma whose nature and whose quality is knowledge.