"Hair typing" is made up of numbers and letters. The most common system is Fia's hair typing system, which was made up back when the internet was young.
Of course that's all weird and funny, but the information is actually quite useful. If you know the system, you can tell a lot of the beneficial hair care for the owner.
It looks something like this: 1BCiii
The first part of the typing, is a number and a letter.
The number goes 1-4 and describes how overall straight or curly your natural hair is.
The number that follows: A,B and C (Except for type 4) narrows the straightness or wave down a bit more.
But that one is less important.
What gives hair its wave or lack thereof, is literally its shape in cross section.
Straight hair is perfectly round and the higher you go up in the curly spectrum, the more oval/flatter the cross section will be.
What can you do with this information?
It tells us that straight hair has less surface to the volume, which if we go for basic math and physics, tells us that this is the strongest shape.
But it also tells us that it will be easier to introduce beneficial treatments into the curlier types, where the is shorter to the inner cortex.
This also means that moisture will evaporate easier from curly types, because it has more surface to the volume.
Straighter types are stronger, but more difficult to deep condition.
Curlier types are easier to deep condition, but needs more moisture.
The second letter in the type, is either F, M or C.
F: Fine hair
M: Medium hair
C: Coarse hair
This is a simplified cross-section of a hair:
Coarse hair type has an extra layer in its structure.
This gives coarse hair a very distinct feature:
It makes noise when you roll it between your fingers. A sort of "crunchy" noise.
Do no mistake this for damage!
Those with coarse hair might also have coarse eyebrows and body hair.
The extra layer makes it difficult for treatments to penetrate deeper into the hair. Those with coarse hair can be very sensitive to build up because the ingredients just attaches to the cuticles instead of penetrating.
Especially oil and proteins can be troublesome for those with coarse hair.
There is no structural difference between fine and medium hair, but fine hair can be extremely fine. There are those who say they can barely see their individual hairs due to how fine they are. Of course this tells us that such fine hair can't take a lot of abuse.
Those with fine hair often dislike cones because it weighs their hair down.
Fine hair often dislikes cones.
Coarse hair often dislikes proteins and oil, deep treatments are often pointless.
This doesn't mean volume as in the total volume of your hairs, but rather how many much volume you have with the individual hairs on your head. Those with fine hair doesn't necessarily have little volume and those with coarse hair doesn't necessarily have a lot of volume.
The overall volume of hair is measured like this:
i: Less than 2 inches/5 centimeters circumference
ii: Between 2-4 inches or 5-10 centimeters circumference
iii: More than 4 inches/10 centimeters circumference
When the hair is compressed as tightly as possible.
Usually this only gives a hint of when you can do certain updos:
For instance with the classic cinnamon bun, those with i-type can do it around BSL length where those with iii-type have to wait until around hip length.
Because the hair has to go around itself, having thick hair means you have to grow thick hair longer than thin to get a updo to work.
The thickness or volume also have an ability to amplify the effects of the straightness and structure. For instance, having very curly and thin hair will make your hair even more sensitive to damage because you have less hairs that can take the damage. But if you have thin hairs and a lot of them, you probably wont be as concerned with damage because you have lots of hairs anyways.
Hair with less volume can amplify hair that requires more care
Hair with more volume can make your hair less sensitive to damage
But in the end, the only real difference is how much length you need to make a certain updo