Merge pull request #176523 from duongau/patch-4

Azure CDN how caching works - add note about cache duration

Author: PRMerger5

articles/cdn/cdn-how-caching-works.md

@@ -13,7 +13,7 @@ ms.workload: tbd
 ms.tgt_pltfrm: na
 ms.devlang: na
 ms.topic: article
-ms.date: 04/30/2018
+ms.date: 10/19/2021
 ms.author: allensu
 
 ---
@@ -130,6 +130,10 @@ The following table describes the default caching behavior for the Azure CDN pro
 
 **CDN cache duration**: Specifies the amount of time for which a resource is cached on the Azure CDN. However, if **Honor origin** is Yes and the HTTP response from the origin server includes the cache-directive header `Expires` or `Cache-Control: max-age`, Azure CDN uses the duration value specified by the header instead. 
 
+> [!NOTE]
+> Azure CDN makes no guarantees about minimum amount of time that the object will be stored in the cache. Cached contents may be evicted from the CDN cache before they are expired if the contents are not requested as frequently to make room for more frequently requested contents.
+> 
+
 ## Next steps
 
 - To learn how to customize and override the default caching behavior on the CDN through caching rules, see [Control Azure CDN caching behavior with caching rules](cdn-caching-rules.md).