For a long time, most major OGC WFS servers have supported stored queries: ArcGIS, Degree, GeoServer, MapServer. In many cases, for performance reasons, WFS servers will not let you query more than one feature type at a time unless you define how the 2 are to be joined. Because these joins are hard to define, one of the easiest approaches is to define the join within a stored query. For example - give me all the parcels and roads in the town of Spuzzum.
The only way to execute a stored query from FME's WFS reader now is to include the stored query parameters on the initial dataset getCapabilities request url, for example:
http://gisserver.domain.com:6080/arcgis/services/SampleWorldCities/MapServer/WFSServer?service=WFS&version=2.0.0&REQUEST=getFeature&TypeName=cities?tIndex=0&count=5&STOREDQUERY_ID=urn:StoredQueries:AttributeDemo&myrank=3
Any parameters you add to the wfs request url eventually get passed to the getFeature request. But this isnt very intuitive so it would be nice to have this added to the WFS parameters user interface. Heres a good link describing most basic types of stored queries that it would be good to be able to support: https://enterprise.arcgis.com/en/server/10.3/publish-services/linux/communicating-with-a-wfs-service-in-a-web-browser.htm
