Wa is the twentieth of the thirty consonants and the odd member of the recitation. It writes a labial approximant, the w of English wet, and it stands at the end of the fifth line after the three affricates, which is the position an added letter would naturally take.
It is also one of the four subscripts. Written beneath a root letter it is called the wa zur, and in that position it can sit under a large number of letters. In most modern pronunciation it makes no audible difference to the syllable at all, so a stack containing it is spoken as though it were not there, and its effect is on the spelling and the identity of the word rather than on the sound.
That double existence is why the grammarians do not fully agree about it. Some accounts treat wa as a letter of the alphabet that also happens to serve as a subscript, in the way that ya, ra and la do. Others treat it as a subjoined form promoted to letterhood, a Tibetan device rather than an inherited letter, and point to its position at the end of a line as the mark of an addition. The atlas records the correspondence to the Indic labial semivowel as debated for that reason, rather than asserting a descent that is not agreed.
Whichever account is preferred, the practical facts are not in dispute. Wa is recited as the twentieth letter, it can head a syllable, and it can be subjoined beneath one. The disagreement is about where it came from, and it changes nothing a reader has to do.