An identifier is a token that identifies a single resource; a resource may have one or more identifiers.
In NetKernel, at the lowest level of implementation, there is no hard-coded, preconceived idea of what constitutes a useful identifier; in resource oriented computing an identifier is an opaque token. Practically, it is useful to use resource identifiers with a consistent, readily parsed and constructed syntax that encodes meaningful semantics as this aids humans readability and comprehension.
Whilst nothing prevents you from creating and using your own identifier syntax, NetKernel uses the Universal Resource Identifier (URI), the industry standard for identifying Internet resources (IETF RFC 3986), as its standard user-side identifier model. The URI is used in standard module spaces and with supplied libraries of tools and services.