diff --git a/schemas/fcmaterials_capi-README.md b/schemas/fcmaterials_capi-README.md
index 357da91..a76f136 100644
--- a/schemas/fcmaterials_capi-README.md
+++ b/schemas/fcmaterials_capi-README.md
@@ -1,8 +1,8 @@
 # EDDN FCMaterials Schema
 
 ## Introduction
-This is the documentation for how to take data from an ED `FCMaterials.json`
-file and properly structure it for sending to EDDN.
+This is the documentation for how to take data from an the ED CAPI `/market`
+endpoint and properly structure it for sending to EDDN.
 
 Please consult [EDDN Schemas README](./README-EDDN-schemas.md) for general
 documentation for a schema such as this.
@@ -16,23 +16,63 @@ to report any such anomalies you find so that we can check and resolve the
 discrepancy.**
 
 ## Senders
-The data source for this schema is the file `FCMaterials.json`.  That it has
-been freshly written is signalled by the ED Journal event `FCMaterials`.
-**NB: This schema is not, currently, for sending CAPI `/market`-sourced data
-about these materials.**
+The data source for this schema is the `/market` ED CAPI endpoint.  You only
+want selected parts of the full data returned for this schema.
 
-So, monitor the Journal as normal, and when you see a `FCMaterials` event open
-the `FCMaterials.json` file for reading, read it, and close it again.  Use the
-data you got from reading this file, not merely the Journal event.
+You **MUST NOT** construct the message by starting with the entirety of the
+CAPI data and then removing everything but what you need.  That risks Frontier
+adding more data to the endpoint and your `fcmaterials_capi` messages being
+rejected as invalid.  Instead, construct the message content by setting just
+the data that is necessary.
 
-Your `message` should primarily be the contents of this file, with the addition
-of any augmentations, as noted below.
+Your `message` object **MUST**:
+1. Have an `"event":"FCMaterials"` member to aid Listeners who pass this
+   through a "usually from the Journal" code path.
+2. Set a `"MarketID"` key with the value from `"id"` in the CAPI data.
+3. Set a `"CarrierID"` key with the value from the `"name"` in the CAPI data.
+4. Set the `"Items"` key's contents directly from the `/market` -> `orders`
+   -> `onfootmicroresources` CAPI data.
+5. Remove any data where the key is `"locName"` from the `"Items"` data.
+
+You **MUST NOT**:
+1. Attempt to set a `"CarrierName"` from any source, the `CarrierID` is
+   sufficient.
+
+### Example algorithm
+
+1. Make a CAPI `/market` query.
+2. Set `event`, `MarketID` and `CarrierID` as outlined above.
+3. Set `Items` value to the data in `orders.onfootmicroresources`.
+4. Process the contents of `Items`, removing any data with a key of `locName`.
 
 ### Augmentations
 #### horizons and odyssey flags
 Please read [horizons and odyssey flags](../docs/Developers.md#horizons-and-odyssey-flags)
 in the Developers' documentation.
 
+You **SHOULD** set these flags from the Journal `FileHeader` data if you are
+reasonably sure you have a live game session against which you are performing
+CAPI queries.
+You **MUST NOT** set them otherwise, as e.g. the player could be active in
+the game on another computer, using a different game mode and the CAPI data
+will be for that game mode.
+
 ## Listeners
 The advice above for [Senders](#senders), combined with the actual Schema file
 *should* provide all the information you need to process these events.
+
+Do note that the data source for this is the CAPI, and as such the data is not
+the same as for the `fcmaterials_journal` schema:
+
+1. There is no good source of `CarrierName` in CAPI `/market` endpoint data, so 
+   that is not included.
+2. The `sales` and `purchases` values do **not** contain the same form of data.
+3. The `sales` member of `Items` will be `[]` if there are no sales orders, but
+   when there are orders:
+    1. It will be an object/dictionary.
+    2. The keys are the commodity ID.
+    3. The value of that key is the rest of the data for that sales order.
+4. The `purchases` value:
+    1. Is always an array, unlike `sales`.
+    2. As a consequence does **not** provide the commodity id at all, only
+      the name.