The Affinity(First) can be chosen by the Player while anymore the player receives can be random(spinner) or the author can choose. The mastery of the affinity is determined by a percentage, their are six main percentages to an affinity:
1. 0% You have no understanding of the element and aren't familiar with it at all, Civilian Level.
2. 20% You have some understanding of the element but not enough to use it correctly, Genin Level.
3. 40% You aren't a beginner but you aren't as versed as someone of higher level, Chunin Level.
4. 60% You know what elements work well with yours and can combine them to make them stronger, Jounin Level.
5. 80% you are nearing Master Level but you aren't there yet, Kage Level.
6. 100% you are one with the element and have full control over it, Master Level.