[APIM] Update to subscription concepts

Author: dlepow

articles/api-management/api-management-subscriptions.md

@@ -7,14 +7,17 @@ author: dlepow
 
 ms.service: api-management
 ms.topic: conceptual
-ms.date: 12/16/2022
+ms.date: 04/19/2023
 ms.author: danlep
 ms.custom: engagement-fy23
 ---
 # Subscriptions in Azure API Management
 
 In Azure API Management, *subscriptions* are the most common way for API consumers to access APIs published through an API Management instance. This article provides an overview of the concept.
 
+> [!NOTE]
+> An API Management subscription is used specifically to call APIs through API Management. It's not the same as an Azure subscription.
+
 ## What are subscriptions?
 
 By publishing APIs through API Management, you can easily secure API access using subscription keys. Developers who need to consume the published APIs must include a valid subscription key in HTTP requests when calling those APIs. Without a valid subscription key, the calls are:
@@ -88,6 +91,11 @@ API publishers can [create subscriptions](api-management-howto-create-subscripti
 
 When created in the portal, a subscription is in the **Active** state, meaning a subscriber can call an associated API using a valid subscription key. You can change the state of the subscription as needed - for example, you can suspend, cancel, or delete the subscription to prevent API access.
 
+## Use a subscription key
+
+To use an API Management subscription key, a subscriber adds the **Ocp-Apim-Subscription-Key** HTTP header to the request, passing the value of the subscription key. Alternatively, include the **Ocp-Apim-Subscription-key** query parameter and value in the request. The query parameter is checked only if the header isn't present.
+
+
 ## Enable or disable subscription requirement for API or product access
 
 By default when you create an API, a subscription key is required for API access. Similarly, when you create a product, by default a subscription key is required to access any API that's added to the product. Under certain scenarios, an API publisher might want to publish a product or a particular API to the public without the requirement of subscriptions. While a publisher could choose to enable unsecured (anonymous) access to certain APIs, configuring another mechanism to secure client access is recommended.