There needs to be an official thread about lengths but here's my understanding on measuring.
For measurements, keep your head in a natural relaxed position... not tilted to the side, back, or front. Also face straight ahead.
Ear Length (EL) - Hair stops at your ears
Chin Length (CL) - Hair stops at your chin
Neck Length (NL) - Can possibly be CL if your hair reaches your neck
Collarbone Length (CBL) - Hair that touches your collarbone
Shoulder Length (SL) - Hair that reaches the line if you drew a line from shoulder to shoulder (top shoulder edges)
Armpit Length (APL) - Hold your arms STRAIGHT DOWN and draw a line from the top of your pit fold to the other.
Bra strap Length (BSL) - Hair that reaches the bottom of your shoulder blade (BSBL)
Mid back Length (MBL) - A measurement half way between your (BSBL) and your waist
Waist Length (WL) - The smallest width of where your sides dip in
Tailbone Length (TBL) - Where your hair touches your tailbone
Hip Length (HL) - Hair reaching the widest part of your hips
Some people measure by their shortest layer, longest layer, hair at a blunt state, or with a few hairs touching it.