Porosity
Low porosity hair
In low porosity, the hair cuticles are actually almost closed. This usually occurs in hair that has never or rarely had any contact with chemicals and extreme heat.
Medium porosity hair
The golden mean. When you have medium porous hair, you are in between high and low. The hair is often healthy and does not need much care.
High porosity hair
In hair with high porosity, the hair cuticles (outer layer of your hair) are wide open and have holes in them.
The Curly Book
So you can vary a lot, eventually you will find out which method your curls respond to best.
If you want to read everything again, the Curl Book is packed with information about the CG method.