It is a part of the ITU-T (International Telecommunication Union - Telecommunication Standardization Sector) recommendations and defines the format of telephone numbers. 

• Number Structure: E.164 numbers can have a maximum of 15 digits and are generally written in the international format, which includes the country code (1 to 3 digits), an optional area code (national destination code), and a local phone number (subscriber number).

• Global Reach and Uniformity: The E.164 standard provides a uniform numbering scheme that ensures telephone numbers are uniquely defined and can be dialed the same way from anywhere in the world.

• Country Codes: The standard assigns country codes to each country. For example, the United States and Canada have the country code +1, the United Kingdom +44, and India +91.

• Use in Modern Technologies: In addition to traditional telephony, E.164 numbers are used in various modern communication technologies and applications, such as VoIP services and some forms of instant messaging, which require a standardized phone number format.

The E.164 standard plays a critical role in the interoperability of global telecommunication systems, providing a consistent framework for numbering phone lines across different countries and technologies.