diff --git a/Volume-Server-API.md b/Volume-Server-API.md
index df9ac5f..e8a085c 100644
--- a/Volume-Server-API.md
+++ b/Volume-Server-API.md
@@ -46,7 +46,7 @@ The size returned is the size stored on SeaweedFS, sometimes the file is automat
 | Content-Type | use the specified content type | "application/octet-stream" |
 | Content-MD5  | verify the uploaded content by MD5 | empty |
 | Authorization | Json Web Token for writes issued by master | empty |
-| Seaweed-xxxxx | Any key-value pair with custom `Seaweed-` prefix. All the custom key-value data are stored as json, should be less than 64KB. | empty |
+| Seaweed-xxxxx | Any key-value pair with custom `Seaweed-` prefix. All the custom key-value data are stored as json, should be less than 64KB. The key will trim out the `Seaeweed-` prefix and then convert into a canonical header key. See https://golang.org/pkg/net/http/#CanonicalHeaderKey | empty |