What about a little research on your side?
Anyway: the pre-defined (standard) options for GENRE are thre for the ID3V1 standard where each genre has a defined number from 1 to 256 - and only this one is stored in the tag. Any string that does not match the predefined list is mapped to "Other".
In the ID3V2.3 standard you can input anything you like into the genre field which means that there is no standard anymore. So the only standard that is to be followed is that for V2.3 tags, a string has to be saved and not a number.
You can add custom genres in the File>Options dialogue.