The policies on requirements and restrictions are an agreement between each domain registry - the entity that holds the registration of domain names - and I.C.A.N.N. (International Corporation of Assigned Names and Numbers).
More information regarding responsibilities of domain registrants can be found here:-
There are some rules that, on the whole apply to most domain names. For example, rules relate to the allowed characters that make up a domain name. These may include characters from both standardised and country specific sets. Usual requirements are that characters allowed are a-z and 0-9 and the '-' (hyphen) character, but such requirements may allow specific regional/local language characters.
Other requirements may apply in some cases to domains names that must:-
Top Level Domains (TLDs), Country Code Top Level Domains (ccTLDs) & Second Level Domains (SLDs) have varying registration requirements, and generally include the above character requirements.
Generally, TLDs such as '.com', '.info' and '.mobi' etc have less registration restrictions than more specific ccTLDs or SLDs. TLD extensions can be used to infer a 'global' presence and require less restriction on who can register them, whereas others such as '.uk', '.fr', '.gr' etc, may infer a more localised presence, and may require more restriction over who may register a domain. Each ccTLD may have restrictions that vary compared with other ccTLDs. The same applies for SLDs, such as '.co.uk'. For example there is a restriction that if a business such as a Limited Company (LTD) or School is registering a '.co.uk' domain, then a company or school number is required to be submitted as part of the registration.
If you are having trouble registering a domain name or would like further information, please don't hesitate to get in touch and we'll be able to help.
Otherwise, when you conduct an availability check on a name, information is available and contained in the list against each domain name result displayed.