Rule elements should have a context attribute. A Schematron schema should have assert or report elements inside the rule elements. Assert and report elements should have a test attribute. Rules defining occurrence rules for Schematron elements and their required attributes. Note that for attributes, it is not that the attribute is being tested for existance, but whether it has a value.
Some elements require certain children or attributes.
Other elements require certain parents. Schematron can represent both these kinds of coupling. The element should only appear inside the Schematron element rule. A element should contain a natural language sentence. The element must have a value for the attribute test.