Balise was known as the Swiss Army Knife of SGML. It can act as a parser and validator, transformation programming language, document manipulator, outliner, pretty-printer, and much else. It consists of a high-level programming language which in effect acts as an API to the document. The interface is the command-line compiler/interpreter.

Unusually (uniquely) it also provides access to the DTD, both logically and syntactically, so that the requirements of the DTD can be queried during document processing; for example finding what other element types are valid at the location of the current one. It also provides access to some non-ESIS parts of the document structure, such as the start and end locations of marked sections.