At a further point along the continuum, the semantics are explicit and are expressed in an informal manner, e.g., a glossary or a text specification document. Given the complexities of natural language, machines have an extremely limited ability to make direct use of informally expressed semantics. This is mainly for humans. There are many examples of informal semantics, usually found in text specification documents. • The meaning of tags in HTML such as

, which means second level header; • The meaning of expressions in modeling languages such as UML (Unified Modeling Language) [OMG 2000], and the original specification of RDF Schema [W3C 1999]; • The meaning of terms in the Dublin Core [Weible & Miller 2000]

Typically, the semantics expressed in informal documents are hardwired by humans in working software. Compiler writers use language definition specifications to write compilers. [...] The main disadvantage of implicit semantics is that there is still much room for ambiguity. This decreases one’s confidence that two different implementations (say of RDF Schema) will be consistent and compatible. Implementations may differ in subtle ways.