SCL imposes no lexical roles on names. It allows any name to denote an individual or a relation or both, and to take any number of arguments (including zero). Particular SCL languages may impose such syntactic restrictions on names by the use of headers. In general, an SCL document consists of a body, consisting of a sequence of SCL sentences, and an optional header. The header should provide sufficient information to enable a generic parser for the SCL language to unambiguously determine the logical form of the setences in teh body. If an SCL document is divided into parts for storage or transmission purposes, each part must be provided with a copy of the header. We will use the notation [H,B] for the document with header H and body B.

Particular SCL concrete syntaxes may define idiosyncratic forms of header, and generic SCL syntax frameworks may provide information about the concrete syntax in the header. All SCL languages must permit a particular form of a header which is an SCL document. Headers of this form will be called SCL headers. SCL headers allow syntactic information to itself be expressed in SCL, providing a uniform technique for comparing and combining SCL content expressed using different syntactic or lexical conventions.

SCL headers may include arbitrary SCL sentences, but a particular SCL vocabulary is provided for the purpose of writing SCL headers, called the header vocabulary.All conforming SCL applications must be able to process SCL headers written using the header vocabulary.

In order to define the semantic conditions on the header vocabulary we need to first define what it means to interpret a document. Intuitively, an interpretation I satisfies a document [H,B] just when it satisfies B and also conforms to the syntactic restrictions described in the header. Interpretations which fail to satisfy the header conditions are considered inappropriate, and are not considered to play any semantic role e.g. in determining entailment of one document by another: the truth-value of the body in an inappropriate interpretation is considered to be irrelevant to entailment.