Comments on Example 1: General Solution for Wide Base P-N Junction

Comments

Getting confused near the end.
If 'a' is where x'=0, and 'b' where x=0, then why is
"Jn at the edge of the depletion region in the p-type material...Jn(x=0)"
and
"Jn at the edge of the depletion region in the n-type material...Jn(x'=0)"?

Furthermore, why is
Jn(x'=0) - Jn(x=0) = qG(xn + xp) ??