Ok. Here's what I learned about curl patterns and why I stopped even paying attention to them.
There are three widely recognized curl typing systems (I don't consider the Deva system to be widely recognized). The least known of these is LOIS. It is literally based on which of those letters a strand of hair forms when it dries (ie a piece of shed hair dropped onto a flat surface).
The two systems most referenced here are Andre Walker's system and FIA. NC.com uses a system based on Andre Walker's. This system directly links curl pattern and texture, which causes questions like "I have fine hair but my curls look more 2c than 3a- what's my curl type??" According to this system, 2c hair is coarse in texture and resistant to styling. 3a hair is fine in texture and has a definite S pattern. He also notes that 3a hair tend to have a variety of curl patterns (doesn't everyone?!). Various incarnations of the curl types on NC.com have given the sidewalk chalk curl diameter for 3a, have stated that type 3 hair has definite root curl where type 2 does not, and once said that it's very easy for type 3a hair to look like 2c hair but very difficult to make 2c hair look like 3a hair without serious time, effort and manipulation. Then, to throw a wrench in the works, there's also the guideline that gets thrown around that says 3a hair needs a good amount of length (shoulder length or longer) to show its true pattern. If this isn't enough to make your head spin, remember that I still have one more system to cover...
FIA uses the same letter and number combos as Andre Walker's system, but in this case, it refers only to curl pattern. Separate characters are used to identify texture and density. According to FIA, 2c hair has a definite S shape, with the occasional full ringlet/curl mixed in. 3a is large bouncy curls, and 3b is defined ringlets. Note the lack of curl circumference requirements.
All of this for something that can change with the weather and really makes absolutely no difference in product selection... So that's my rant for the day. Hopefully this helps a bit