Plastic J significance in CTOD equation

I am working on the validation of CTOD.

In the evaluation of CTOD using the
J-integral approach as applied in ASTM E1290 and E1820, the plastic J
component is defined as (Npl*Apl)/(Bn*Bo)
Npl=dimensionless constant(plasticity)
Apl=area under load vs. displacement plot
Bn=ligamen length (W-a)
Bo=section thickness

On E1820 there are different functions provided for Npl if Apl is
evaluated differently (CMOD or load-line displacement).

Do anybody know what are the relation of the Npl to Apl and how they interact
for the J-integral evaluation?