Merge pull request #42685 from pegasas/quota

clarification for ResourceQuota concept

Author: k8s-ci-robot

content/en/docs/concepts/policy/resource-quotas.md

@@ -149,8 +149,8 @@ Refer to [Logging Architecture](/docs/concepts/cluster-administration/logging/)
 
 ## Object Count Quota
 
-You can set quota for the total number of certain resources of all standard,
-namespaced resource types using the following syntax:
+You can set quota for *the total number of one particular resource kind* in the Kubernetes API,
+using the following syntax:
 
 * `count/<resource>.<group>` for resources from non-core groups
 * `count/<resource>` for resources from the core group
@@ -168,16 +168,21 @@ Here is an example set of resources users may want to put under object count quo
 * `count/jobs.batch`
 * `count/cronjobs.batch`
 
-The same syntax can be used for custom resources.
+If you define a quota this way, it applies to Kubernetes' APIs that are part of the API server, and
+to any custom resources backed by a CustomResourceDefinition. If you use [API aggregation](/docs/concepts/extend-kubernetes/api-extension/apiserver-aggregation/) to
+add additional, custom APIs that are not defined as CustomResourceDefinitions, the core Kubernetes
+control plane does not enforce quota for the aggregated API. The extension API  server is expected to
+provide quota enforcement if that's appropriate for the custom API.
 For example, to create a quota on a `widgets` custom resource in the `example.com` API group, use `count/widgets.example.com`.
 
-When using `count/*` resource quota, an object is charged against the quota if it exists in server storage.
+When using such a resource quota (nearly for all object kinds), an object is charged
+against the quota if the object kind exists (is defined) in the control plane.
 These types of quotas are useful to protect against exhaustion of storage resources.  For example, you may
 want to limit the number of Secrets in a server given their large size. Too many Secrets in a cluster can
 actually prevent servers and controllers from starting. You can set a quota for Jobs to protect against
 a poorly configured CronJob. CronJobs that create too many Jobs in a namespace can lead to a denial of service.
 
-It is also possible to do generic object count quota on a limited set of resources.
+There is another syntax only to set the same type of quota for certain resources.
 The following types are supported:
 
 | Resource Name | Description |
@@ -197,6 +202,8 @@ created in a single namespace that are not terminal. You might want to set a `po
 quota on a namespace to avoid the case where a user creates many small pods and
 exhausts the cluster's supply of Pod IPs.
 
+You can find more examples on [Viewing and Setting Quotas](#viewing-and-setting-quotas).
+
 ## Quota Scopes
 
 Each quota can have an associated set of `scopes`. A quota will only measure usage for a resource if it matches