diff --git a/core/consensus/src/util.rs b/core/consensus/src/util.rs
index f56b517f3..9e7959c14 100644
--- a/core/consensus/src/util.rs
+++ b/core/consensus/src/util.rs
@@ -38,6 +38,8 @@ pub fn time_now() -> u64 {
 /// `keccak256` hash calculation at the end of each height. The reason why not
 /// use address directly is that the `PeerId` is binding with public key not
 /// address.
+/// The `common_ref` do not affect the signature verification, it is a
+/// placeholder.
 pub struct OverlordCrypto {
     private_key: BlsPrivateKey,
     addr_pubkey: RwLock<HashMap<Bytes, BlsPublicKey>>,
diff --git a/core/run/src/lib.rs b/core/run/src/lib.rs
index 8e0c62439..ad5257955 100644
--- a/core/run/src/lib.rs
+++ b/core/run/src/lib.rs
@@ -368,6 +368,7 @@ fn init_crypto(
         bls_pub_keys.insert(address, pub_key);
     }
 
+    // The `common_ref` is a placeholder, use empty string.
     let crypto = OverlordCrypto::new(bls_priv_key, bls_pub_keys, String::new());
     Ok(Arc::new(crypto))
 }