The constant evolution of descriptive means led to the Extended Backus Naur Form
The Augmented Backus Naur Form is compact, relatively simple and understandable. In the early years, the Arpanet had different ABNF definitions for e-mail, for example. In later documents
, these definitions were separated to ensure uniform rules. Unlike the classic Backus Naur Form (BFN), the Augmented version introduced naming rules, repetitions, alternatives, and value ranges. It is described in RFC 4234.