The following categories are defined by the XProc specification itself:
XProc steps that a conformant XProc processor must support. In other words: whatever processor you use for processing your XProc pipelines, you can trust that these steps will always work.
XProc dynamic pipeline execution steps
XProc steps that deal with dynamic execution of pipelines (running pipelines inside another pipeline).
XProc steps that deal with email. An XProc processor is not required to support these.
XProc file and directory related steps
XProc steps that deal with files and directories. An XProc processor is not required to support these.
XProc Invisible XML related steps
XProc steps that deal with the processing of Invisible XML. An XProc processor is not required to support these.
XProc operating system related steps
XProc steps that deal with the operating system. An XProc processor is not required to support these.
XProc paged media related steps
XProc steps that deal with creating paged media, for instance PDF. An XProc processor is not required to support these.
XProc steps that deal with text, for instance Markdown. An XProc processor is not required to support these.
XProc validation related steps
XProc steps that deal with validation of XML and JSON documents. An XProc processor is not required to support these.
The following categories are defined by XProcRef:
Steps that implement additional standards.
These steps handle archives (for instance, ZIP files).
These steps act on or use the base URI value of elements.
These steps implement basic XML manipulation, like adding attributes, inserting elements, etc.
Steps that have to do with compressing (or uncompressing) documents.
Interaction with the environment
Steps that interact (or prepare interaction) with the environment of the pipeline, for instance communication over HTTP(S) or working with directories and files.
Steps for handling JSON documents.
Miscellaneous steps, which are usually used for housekeeping purposes in the pipeline.
Steps that have to do with namespace handling.
Steps that do something with (lines in) text documents.