Some ArcGIS Server FeatureServices serve Relationship Classes, similar to Relationship Classes in FileGDBs.

These don’t show up in the Esri ArcGIS Server Feature Service Reader, however. It would be very convenient if FME would support these Relationship Classes, as some FeatureServices don’t have foreign keys in their tables. In such cases the Relationship Class is the only way to join tables.

Relationship Classes are listed in [ArcGIS Server URL]/FeatureServer/relationships.

For example: ttps://basisregistraties.arcgisonline.nl/arcgis/rest/services/BAG/BAGv3/FeatureServer/relationships