diff --git a/api-ref/source/allocations.inc b/api-ref/source/allocations.inc
index fbd8dea99..82315d2c2 100644
--- a/api-ref/source/allocations.inc
+++ b/api-ref/source/allocations.inc
@@ -28,8 +28,14 @@ Normal response codes: 204
 Error response codes: badRequest(400), conflict(409)
 
 * `409 Conflict` if there is no available inventory in any of the
-  resource providers for any specified resource classes or inventories
-  are updated by another thread while attempting the operation.
+  resource providers for any specified resource classes.
+* `409 Conflict` with `error code <error_codes_>`_
+  ``placement.concurrent_update`` if inventories are updated by another request
+  while attempting the operation. See :ref:`generations`.
+* `409 Conflict`  with `error code <error_codes_>`_
+  ``placement.concurrent_update`` at microversion 1.28 or higher if allocations
+  for a specified consumer have been created, updated, or removed by another
+  request while attempting the operation. See :ref:`generations`.
 
 Request
 -------
@@ -122,8 +128,14 @@ Normal Response Codes: 204
 Error response codes: badRequest(400), itemNotFound(404), conflict(409)
 
 * `409 Conflict` if there is no available inventory in any of the
-  resource providers for any specified resource classes or inventories
-  are updated by another thread while attempting the operation.
+  resource providers for any specified resource classes.
+* `409 Conflict` with `error code <error_codes_>`_
+  ``placement.concurrent_update`` if inventories are updated by another request
+  while attempting the operation. See :ref:`generations`.
+* `409 Conflict`  with `error code <error_codes_>`_
+  ``placement.concurrent_update`` at microversion 1.28 or higher if allocations
+  for the specified consumer have been created, updated, or removed by another
+  request while attempting the operation. See :ref:`generations`.
 
 Request (microversions 1.12 - )
 -------------------------------
diff --git a/api-ref/source/errors.inc b/api-ref/source/errors.inc
index 4419162c0..620cfb2f5 100644
--- a/api-ref/source/errors.inc
+++ b/api-ref/source/errors.inc
@@ -30,6 +30,7 @@ error response is defined by the OpenStack errors_ guideline.
           the above because the error is produced by code other than the
           placement service.
 
+.. _`error_codes`:
 
 Error Codes
 ===========
diff --git a/api-ref/source/generations.inc b/api-ref/source/generations.inc
new file mode 100644
index 000000000..be3a0ece9
--- /dev/null
+++ b/api-ref/source/generations.inc
@@ -0,0 +1,48 @@
+.. _generations:
+
+==========================================
+Resource Provider and Consumer Generations
+==========================================
+Placement handles concurrent requests against the same entity by maintaining a
+**generation** for resource providers and consumers. The generation is an
+opaque value that is updated every time its entity is successfully changed on
+the server.
+
+At appropriate microversions, the generation is returned in responses involving
+resource providers and/or consumers (allocations), and must be included in
+requests which make changes to those entities. The server checks to make sure
+the generation specified in the request matches the internal value. A mismatch
+indicates that a different request successfully updated that entity in the
+interim, thereby changing its generation. This will result in an HTTP 409
+Conflict response with `error code <error_codes_>`_
+``placement.concurrent_update``.
+
+Depending on the usage scenario, an appropriate reaction to such an error may
+be to re-``GET`` the entity in question, re-evaluate and update as appropriate,
+and resubmit the request with the new payload.
+
+The following pseudocode is a simplistic example of how one might ensure that a
+trait is set on a resource provider.
+
+.. note:: This is not production code. Aside from not being valid syntax for
+          any particular programming language, it deliberately glosses over
+          details and good programming practices such as error checking, retry
+          limits, etc. It is purely for illustrative purposes.
+
+::
+
+ function _is_concurrent_update(resp) {
+     if(resp.status_code != 409) return False
+     return(resp.json()["errors"][0]["code"] == "placement.concurrent_update")
+ }
+
+ function ensure_trait_on_provider(provider_uuid, trait) {
+     do {
+         path = "/resource_providers/" + provider_uuid + "/traits"
+         get_resp = placement.GET(path)
+         payload = get_resp.json()
+         if(trait in payload["traits"]) return
+         payload["traits"].append(trait)
+         put_resp = placement.PUT(path, payload)
+     } while _is_concurrent_update(put_resp)
+ }
diff --git a/api-ref/source/index.rst b/api-ref/source/index.rst
index c356454ed..8919e95a3 100644
--- a/api-ref/source/index.rst
+++ b/api-ref/source/index.rst
@@ -16,6 +16,7 @@ header for APIs sending data payloads in the request body (i.e. ``PUT`` and
 
 .. include:: request-ids.inc
 .. include:: errors.inc
+.. include:: generations.inc
 .. include:: root.inc
 .. include:: resource_providers.inc
 .. include:: resource_provider.inc