Updated: Apr 21, 2020
There are no set rules when writing a knitting pattern that tells you which type of bracket to use for what. It is up to the writer and their personal preference, this is usually influence by who taught them how to write patterns or who they work for. So this is how brackets work in my patterns.
There are two types of brackets, the numbers within, or one to the left, of ether type of bracket, corresponds to the different sizes that the pattern can be knitted.