It consists of just two letters: C (for spine) and M (for seat).
|Photo on the left from Kim Brooks of Breathe In Photography and on the right by Jenna Sparks Photography|
When choosing a carrier for a baby under 12 months it is recommended that babywearers look for one that allows the baby to sit in the carrier with a C shaped spine (see left pic). (After 12 months babies start to straighten up and tend to sit more upright so generally you will not see the C shaped spine with a baby past the 12 month mark.)
Looking at how your bub is seated in a carrier is another way to check the carrier's ergonomic standard. Can they sit in an M seat, with their knees higher than their bum? They may be sitting with their legs out (as in right pic) or with their legs inside the carrier if younger than 3-4 months but both positions should offer babe an M seat.
Both the C shaped spine and the M seat are great positions to encourage natural physical development of the spine and hips. All ergonomic carriers should offer these two letters for your baby!