If you're a flatback, your entire back should touch the floor if you lay on your back.

If you're hyperextended, that means you have your shoulders back and down and tailbone up and back, with much curvature in the low back but little in the mid back. This is fairly common among runners and computer mouse users.

If you're hyperflexed, that means you have little curvature in your lower back (i.e., tailbone forward and down) while having your shoulders up and forward. Hyperflexed can easily touch their toes without bending their knees. This is common among excessive smartphone users.

Bicameral is my own name for the family of postures, and it means one has shoulders up and forward and tailbone up and back simultaneously. The most common way to get there is to sleep on your chin/stomach.

I warn you not to try anything too drastic in regards to this.