Aside from the obvious variety of curls we all have on our heads, much of the confusion about curl pattern types is because there are two different typing systems that utilize the letter-number combinations, but their meanings are different from one system to the other (you might see that some people will note next to their curl pattern in their signature either "FIA" or "NC.com").
The curl typing system that's used by NC.com is Andre Walker's system. His definitions of curl pattern also take hair texture into account. According to this system, 2c hair is coarse and difficult to style, where 3a hair is fine in texture and can be easily straightened. He also states that type 3a hair has root curl, which is not found in 2c hair, and that while it's very easy to get 3a hair to look like 2c hair, it's very difficult to make 2c hair look like 3a hair without major work or curlers. It's this system that uses the comparison of sidewalk chalk for the circumference of 3a ringlets (or a sharpie marker for 3b).
FIA uses the same letters and numbers as Andre Walker, but in the FIA system, this only refers to curl pattern. According to FIA, 2c is a head full of strong waves/wurls, with a few curls mixed in. 3a is a head full of large soft curls/ringlets. Texture and density are identified separately, thus the FIA classification is 3 different characters.
As Knotty Maddy mentioned, curl pattern doesn't really impact anything (except cuts/updos). Properties (texture, porosity, etc) are what will determine the types of products that will work best on your hair and when use the right products and treat your hair well, you will maximize your natural curl pattern, whatever it is.
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3a/f/iii Before the chop- Donated to Locks of Love, January 2013
Modified CG since 11/5/11
CO-WASH: VO5 Vanilla Mint Tea Clarifying, DevaCare No-Poo
CLARIFY: CHS Treatment Shampoo (used prior to PT/DT)
RO: DevaCare One Condition, Regis Olive Oil
LI: Cure Care, As I Am Leave-In
STYLE: Re:Coil, Curl Keeper, Biotera Gel, Deva Ultra Defining Gel, GVP Liquid Sculpting Gel, Curls Rock Amplifier and Strong Hold Mousse
PT/DT: IAGirl's PT, One n' Only Argan Hydrating Mask