I have been preparing for a exam on power electronics for quite some time.

I cannot understand some things:

what does the subscripts AO and 1 refer to at (u_{A0})_{1}?Thanks.

 

HiSG,
This link may be helpful.
E
https://math.stackexchange.com/questions/123699/what-do-subscripted-numbers-in-an-equation-mean