diff --git a/google/cloud/securitycenter/v1/securitycenter_service.proto b/google/cloud/securitycenter/v1/securitycenter_service.proto
index 6c368bfd..a8a05993 100644
--- a/google/cloud/securitycenter/v1/securitycenter_service.proto
+++ b/google/cloud/securitycenter/v1/securitycenter_service.proto
@@ -842,6 +842,11 @@ message UpdateFindingRequest {
 
   // The FieldMask to use when updating the finding resource. This field should
   // not be specified when creating a finding.
+  //
+  // When updating a finding, an empty mask is treated as updating all mutable
+  // fields and replacing source_properties.  Individual source_properties can
+  // be added/updated by using "source_properties.<property key>" in the field
+  // mask.
   google.protobuf.FieldMask update_mask = 2;
 }
 
@@ -851,6 +856,8 @@ message UpdateOrganizationSettingsRequest {
   OrganizationSettings organization_settings = 1;
 
   // The FieldMask to use when updating the settings resource.
+  //
+  //  If empty all mutable fields will be updated.
   google.protobuf.FieldMask update_mask = 2;
 }
 
@@ -860,6 +867,8 @@ message UpdateSourceRequest {
   Source source = 1;
 
   // The FieldMask to use when updating the source resource.
+  //
+  // If empty all mutable fields will be updated.
   google.protobuf.FieldMask update_mask = 2;
 }
 
@@ -869,8 +878,14 @@ message UpdateSecurityMarksRequest {
   SecurityMarks security_marks = 1;
 
   // The FieldMask to use when updating the security marks resource.
+  //
+  // The field mask must not contain duplicate fields.
+  // If empty or set to "marks", all marks will be replaced.  Individual
+  // marks can be updated using "marks.<mark_key>".
   google.protobuf.FieldMask update_mask = 2;
 
   // The time at which the updated SecurityMarks take effect.
+  // If not set uses current server time.  Updates will be applied to the
+  // SecurityMarks that are active immediately preceding this time.
   google.protobuf.Timestamp start_time = 3;
 }