diff --git a/cs3/storage/provider/v1beta1/provider_api.proto b/cs3/storage/provider/v1beta1/provider_api.proto
index 8b02a7b..ebb980a 100644
--- a/cs3/storage/provider/v1beta1/provider_api.proto
+++ b/cs3/storage/provider/v1beta1/provider_api.proto
@@ -75,8 +75,11 @@ service ProviderAPI {
   // Initiates the download of a file using an
   // out-of-band data transfer mechanism.
   rpc InitiateFileDownload(InitiateFileDownloadRequest) returns (InitiateFileDownloadResponse);
-  // Initiates the upload of a file using an
-  // out-of-band data transfer mechanism.
+  // Initiates the upload of a file using an out-of-band data
+  // transfer mechanism. SHOULD return CODE_FAILED_PRECONDITION
+  // if the reference is already locked with a mismatched lock.
+  // Additionally, the lock check MUST be enforced by the data
+  // transfer protocol returned in response.
   rpc InitiateFileUpload(InitiateFileUploadRequest) returns (InitiateFileUploadResponse);
   // Returns the list of grants for the provided reference.
   // MUST return CODE_NOT_FOUND if the reference does not exists.
@@ -371,7 +374,8 @@ message InitiateFileUploadRequest {
   }
   // OPTIONAL.
   // A lock_id: should the reference exist and be locked, the stored
-  // lock_id MUST be equal to the given value.
+  // lock_id SHOULD be equal to the given value. Additionally, the
+  // check MUST be enforced by the out-of-band transfer protocol.
   string lock_id = 5;
 }