Friends, community members, Dashers of the world: I’m proud to announce the release of DIP 5, which defines rules for registering usernames on the Dash blockchain. We refer to these username accounts as Blockchain Users (BUs). This builds upon DIP 2 and defines several special transaction types with payloads relevant to BUs.
Blockchain Users are the foundation upon which the Dash Platform (including DAP system) is built.
You can read about DIP 5 on GitHub.
edit: @thephez has posted a less-technical overview of DIP 5 titled Introducing blockchain users