Character strings are reserved through the ICANN structure for managing Internet domain names and IP addresses. To avoid chaos on the Internet, it had been decided that country designations as well as other names and abbreviations considered important could not be registered as a two-character domain. That’s why the new gTLDs (generic top-level domains) have become common.
(example)ICANN(Reserved Names for New gTLDs)
ICANN（About Reserved Names）
Character strings made up in part or in whole of abusive or obscene Japanese or English words are the issue. Use of such words is banned. If, however, the meaning of a word in a proposed character string differs from that of a similarly spelled word considered obscene or abusive, the applicant should use the following form to inquire about whether the character string will be acceptable. The registration of a domain name may be canceled if a problem with it comes up later.