Accounting flexfield (chart of account structure) is one key flexfield used in GL. Yes you are right there are tons of other key flexfields used in other modules. Their purpose is obvious from their name. For example Territory KFF records territories and Item KFF records items. You can customize them in any way you like in terms of number of segments and the segment attributes. You can read more about them from the documentation if you like. I will discuss them once we I do the related module.