ISO/IEC 13250:2003 (2^nd edition) specifies two syntaxes for the interchange of Topic Maps. One of these syntaxes is based on the ISO/IEC 10744:1997 (HyTime) meta-DTD (meta Document Type Definition), and it is itself specified as a meta-DTD. The other, called XTM (XML Topic Maps), is specified as an eXtensible Markup Language (XML) DTD.
Both syntaxes allow the expression of:

• associations between subjects of discourse and zero or more of their names, to be used in various contexts,
• associations between subjects and occurrences (pieces of relevant addressable information),
• user-defined associations among any arbitrary subjects of discourse,
• user-defined subjects which are classes of subjects, including classes of associations and the distinct roles that are played in instances of associations, and
• scopes, expressed as sets of subjects, within the context of which specific associations are meant to be understood as valid and/or relevant.