diff --git a/schemas/README-EDDN-schemas.md b/schemas/README-EDDN-schemas.md
index b4651d8..3de98f6 100644
--- a/schemas/README-EDDN-schemas.md
+++ b/schemas/README-EDDN-schemas.md
@@ -268,6 +268,39 @@ changes to your code.
 It is also advisable to Watch this repository on GitHub so as to be aware 
 of any changes to schemas.
 
+#### `horizons` and `odyssey` flags
+
+Where the schema allows for them, add the `horizons` and `odyssey`
+keys with boolean values.  `null` is not allowed in the values, so if
+you cannot determine a value do not include that key at all.  The best
+source of these is the `LoadGame` event from journals.  It's present
+both in the PC local files and the CAPI journal data.  If you're
+composing a shipyard or outfitting message from CAPI data then it is
+possible to synthesise the `horizons` flag.  For now consult the function
+`capi_is_horizons()` in
+[EDMarketConnector:plugins/eddn.py](https://github.com/EDCD/EDMarketConnector/blob/stable/plugins/eddn.py)
+for a method to achieve this.
+
+As of 2022-01-22 the following was observed for the `LoadGame` events as
+present in CAPI-sourced Journal files (which were confirmed to match the
+PC-local files for these events):
+
+- PC Odyssey Client, game version `4.0.0.1002`:
+    ```json
+    { "timestamp":"2022-01-20T11:15:22Z", "event":"LoadGame", "FID":"F<elided>", "Commander":"<elided>", "Horizons":true, "Odyssey":true,...
+    ```
+- PC Horizons Client, game version `3.8.0.403`, no `Odyssey` key was
+   present:
+    ```json
+    { "timestamp":"2022-01-20T11:20:17Z", "event":"LoadGame", "FID":"F<elided>", "Commander":"<elided>", "Horizons":true,...
+    ```
+
+- PC 'base' Client, game version `3.8.0.403`, no `Odyssey` key was
+   present:
+    ```json
+    { "timestamp":"2022-01-20T11:22:54Z", "event":"LoadGame", "FID":"F<elided>", "Commander":"<elided>", "Horizons":false,...
+    ```
+
 ### Server responses
 There are three possible sources of HTTP responses when sending an upload 
 to EDDN.
@@ -299,8 +332,10 @@ make a valid request" responses you might experience the following:
 
 #### bottle responses
 1. `413` - `Payload Too Large` - `bottle` enforces a maximum request size 
-   and the request exceeds that.  As of 2022-01-07 the limit is 1MiB, and 
-   pertains to the plain-text size, not after gzip compression if used.
+   and the request exceeds that.  As of 2022-01-07 the limit is 1MiB,
+   which is versus the compressed size of the body, if compression is
+   used.  Thus compression *will* allow for sending approximately 10x
+   larger messages.
    To verify the current limit check for the line that looks like:
 
       ```