I know it can be very confusing. I will try to make it simple.
Bone Pressed Erect Length or BPEL: It is measured on your excited penis. Put a ruler on your penis and press it against the belly bone. Once done, measure the distance from there to the tip of your penis glans.
Non-Bone Pressed Erect Length or NBPEL: It is measured on your erect penis. Put a ruler on your penis, ensuring the ruler touches your pelvic skin. After that, stretch your penis, and make sure it stays parallel to your ruler. Once done, note the measurement from there to the penile tip.
The major difference between the two is that NBPEL is the visible length and the length used during intercourse.
I hope I am clear.