Variance type

Dear All, 

     When I do miro, I found there is a variance type A or B in the line item in miro.

     What doese this variance type mean, how does it use? What's relationsip with SA or PO

     Does any one can help me on this?

     A:Difference Between GR and IR (PO-Based IR)

     B:Difference Between GR and IR (GR-Based IR)

Thanks!