Clarify behaviour in POST /validation

extensions/federation/README.md

@@ -175,6 +175,18 @@ schema:
 
 **Note:** In Collections this should generally be provided on the top-level of the object.
 
+### Validation
+
+If this property is returned through the `POST /validation` endpoint, it has the meaning as listed below.
+This also covers the case where the federation supports splitting a process into pieces so that different parts can run on different back-ends.
+
+- Endpoint returns *without* errors:
+  - `federation:backends` is included in the response: The listed back-ends support the workflow (either partially if splitting is supported, or in full).
+  - `federation:backends` is *not* included in the response: At least one of the back-ends support the workflow.
+- Endpoint returns errors:
+  - `federation:backends` is included in the response: The listed back-ends were checked and none of the back-ends can run the workflow as is (neither splitted if supported, nor in full).
+  - `federation:backends` is *not* included in the response: None of the back-ends can run the workflow as is.
+
 ### Examples
 
 #### Process
@@ -213,4 +225,5 @@ schema:
   },
   ...
 }
-```
+```
+