No pornographic, sexual or racist words such as sex, sexo, porn, porno, gay, lesbian, etc. Also, domain names like www.bi are not permitted. Restricted domain names will not be activated and the money will not be refunded.
A restricted domain is one of these : gov, net, edu, ac, com, co, int, mil, gouv or any other name related to government or public company use. Contact firstname.lastname@example.org if you want to create such a domain. Note however that it will be assigned only if sponsored by an official authority. Generic names are also discouraged (such as bank, nom, art ... or a geographic notion) as it also misleads people.