The .io domain extension is the
.io Domain Information
Initially serving as the official country-code domain extension for a group of small islands and atolls known as the British Indian Ocean Territory.
.io domains have since become popular in startups and tech companies representing the low level programming and mechanical engineering concept of input / output.
Registrations are allowed under the .IO ccTLD ( country code Top Level Domain) extension directly
IO domain names as a whole are open to register by any individual or organization and do not require you to be a citizen of any specific country.
Originally used exclusively for Indian Ocean Territory domains .io is now promoted for startup and tech companies due to its small length and tech association.