clarify why unknown field names are tolerated

src/main/java/com/conveyal/analysis/controllers/BundleController.java

@@ -134,8 +134,9 @@ private Bundle create (Request req, Response res) {
                 bundle.totalFeeds = bundleWithFeed.totalFeeds;
             }
             if (files.get("config") != null) {
-                // For validation, rather than reading as freeform JSON, deserialize into a model class instance.
-                // However, only the instance fields specifying things other than OSM and GTFS IDs will be retained.
+                // Validation by deserializing into a model class instance. Unknown fields are ignored to
+                // allow sending config to custom or experimental workers with features unknown to the backend.
+                // The fields specifying OSM and GTFS IDs are not expected here. They will be ignored and overwritten.
                 String configString = files.get("config").get(0).getString();
                 bundle.config = JsonUtil.objectMapper.readValue(configString, TransportNetworkConfig.class);
             }

src/main/java/com/conveyal/analysis/models/Bundle.java

@@ -48,7 +48,7 @@ public class Bundle extends Model implements Cloneable {
     public int feedsComplete;
     public int totalFeeds;
 
-    // The definitive TransportNetworkConfig is a JSON file stored alonside the feeds in file storage. It is
+    // The definitive TransportNetworkConfig is a JSON file stored alongside the feeds in file storage. It is
     // duplicated here to record any additional user-specified options that were supplied when the bundle was created.
     // It may contain redundant copies of information stored in the outer level Bundle such as OSM and GTFS feed IDs.
     public TransportNetworkConfig config;

src/main/java/com/conveyal/r5/transit/TransportNetworkCache.java

@@ -168,6 +168,8 @@ private TransportNetworkConfig loadNetworkConfig (String networkId) {
         File configFile = fileStorage.getFile(configFileKey);
         try {
             // Use lenient mapper to mimic behavior in objectFromRequestBody.
+            // A single network configuration file might be used across several worker versions. Unknown field names
+            // may be present for other worker versions unknown to this one. So we can't strictly validate field names.
             return JsonUtilities.lenientObjectMapper.readValue(configFile, TransportNetworkConfig.class);
         } catch (IOException e) {
             throw new RuntimeException("Error reading TransportNetworkConfig. Does it contain new unrecognized fields?", e);