XSLT 2.0 introduced the capability to declare the required types of, among other things, variables. The type can be a built-in or a user-defined type imported from an XML schema. Schematron is currently missing both: The possibility to declare the required type of a variable and to import user-defined data types.

XSLT 3.0 added accumulators as a new mechanism to capture and process data. An accumulator associates a series of values with nodes of the document tree. The value of an accumulator is obtained by calling the respective accumulator functions fn:accumulator-before() and fn:accumulator-after(). Although accumulators are designed for streaming transformations they can also be used with non-streaming transformations.

The addition of user-defined functions in XSLT 2.0 opened up the possibility of reusable, user-defined function libraries. XSLT 3.0 made the notion of user-defined libraries explicit by adding packages, collections of stylesheet modules with a controlled interface.

Schematron lacks a way of including user-defined function libraries other than using schema composition instructions. But neither sch:include nor sch:extends is a good fit. The former inserts the content of an XML document in place of the element, the latter is only allowed to extend a rule. ^[2]