To register a domain name, the objects mentioned in the Registration Form (hosts, Mail Exchangers, or zone Name Servers) must be fully connected to the Internet and properly configured. There is no reservation service for domain names - at least one host must bear the domain name being registered (multiple Domain Names per host are permitted.) You must comply with all relevant RFCs when registering Domain Name.
For Host registration and Mail eXchanger (MX) registration there are no any special requirements, except that hosts being registered must be accessible over the Internet. The number of individual host registrations per domain is limited to 5 (if you exceed this number, you should consider establishing your own Primary and Secondary Name Server and managing your zone yourself). Here is a list of locations which could assist you with establishing a Secondary MX Server.
For zone delegation, requirement is that the Primary Name Server and Secondary Name Server for the zone to be delegated should be fully configured and accessible over the Internet for the registration to take place. You must establish the Secondary Name Server for your zone in the location not sharing the same single connection to the Internet as the Primary Name Server. Here is a list of locations which could assist you with establishing a Secondary Name Server.