diff --git a/object/types.proto b/object/types.proto
index 41b785b..dfc3afc 100644
--- a/object/types.proto
+++ b/object/types.proto
@@ -124,7 +124,8 @@ message Header {
   //   Marks smaller parts of a split bigger object
   // * [ __SYSTEM__EXPIRATION_EPOCH ] \
   //   (`__NEOFS__EXPIRATION_EPOCH` is deprecated) \
-  //   Tells GC to delete object after that epoch
+  //   The epoch after which object with no LOCKs on it becomes unavailable.
+  //   Locked object continues to be available until each of the LOCKs expire.
   // * [ __SYSTEM__TICK_EPOCH ] \
   //   (`__NEOFS__TICK_EPOCH` is deprecated) \
   //   Decimal number that defines what epoch must produce