A compound step, as distinct from an atomic step, is one who’s implementation is determined by the subpipeline it contains.

A dynamic error is one which occurs while a pipeline is being evaluated.

An implementation-defined feature is one where the implementation has discretion in how it is performed. Conformant implementations must document how implementation-defined features are performed.

An implementation-dependent feature is one where the implementation has discretion in how it is performed. Implementations are not required to document or explain how implementation-dependent features are performed.

From the XSLT 3.0 specification, “a selection pattern uses a subset of the syntax for path expressions, and is defined to match a node if the corresponding path expression would select the node. Selection patterns may also be formed by combining other patterns using union, intersection, and difference operators.”

The steps and variables, and the connections between them, that make up the body of a compound step or pipeline.