When writing a campaign setting, certain considerations must be given to the options you are giving to your players when they are creating and growing their character concepts. Your vision of the campaign world may be wide open and permissive, including anything and everything the players bring to the table. It could also be very restrictive, with only a select few options being deemed appropriate for the campaign you want to run.
Most DMs will find their personal style falls somewhere between the two—there are relatively few completely permissive or restrictive DMs out there. When building your campaign world, you need to ask yourself what does and doesn’t fit for anything that could affect a player’s options, and whenever possible, a good reason needs to be provided. Nothing is so frustrating to a player as having a character concept rejected out of hand “because I said so,” after all.