What makes TAG a BY?
That is like asking what makes Bais Yaakov of Boro Park, Bais Yaakov of LA, Bais Yaakov of Minneapolis, a bais yaakov.
The common denominators throughout the bais yaakov system are their rule books and their ideal end products. Uniform, no talking to boys, tznius, good middos, and being taught the majority of your school subjects by Rabbis and rebbitzins.