some questions about result of D-statistics(qpDstat)

Hi !!!
from README.Dstatistics,I read these:

D = num/ den
well, It's not the same as "ABBA-BABA" that I often see

and
If the Z-score is +ve, then the gene flow occured either between W and Y or X and Z
If the Z-score is -ve, then the gene flow occured either between W and Z or X and Y.

what does "+ve" or "-ve" mean? Do these mean >0 or <0 ?

And How do I determine between which two groups the gene flow occurs.
I learned that W, X, Y, Z to satisfy such a relationship: (((W, X), Y), Z), to determine, when D / Z-score greater than 0, and gene flow occurred in the W Between Y, less than 0 occurs between X and Y.

Or I can set Z to be an obvious outgroup, then the phylogenetic relationship above does not need to be satisfied, and I can also determine whether the gene flow occurs between W and Y (D>0) or between X and Y (D <0)

At last，It's about visualization. A fixed WXYZ combination has only one value(D/Z-score), so it cannot draw the D statistical results shown in box plots that I often see.

Start with README.Dstatistics and the qpDstat usage and output documentation. Resolve the questions about D and Z-score signs, group interpretation, and visualization by checking the documented statistical definitions and existing examples. Done means the README clearly explains these points and distinguishes what one fixed WXYZ combination can show.