I would like to point out something which is not explicitely mentioned in this answer, but which has been the way we have used this policy over the past years.
Sam Holder's answer mentions that we don't cover ingredients which are so common that you can easily find recipes for them. We define this as being common somewhere in the world, as opposed to "in ...
Additional advice for people who have had a question closed and been referred to these guidelines:
If you are looking for information on how to use a common ingredient, please see the following question:
How does one find recipes given an ingredient rather than the recipe name?