README add Note and fix something.

README.md

@@ -44,13 +44,18 @@ The 120 columns fall into **6 categories**:
 
 ### ID & Label Columns (5 columns)
 
-| Column | Arrow Type | Nulls 
-|---|---|---|
-| `user_id` | `int64` | 0 | 
-| `item_id` | `int64` | 0 | 
-| `label_type` | `int32` | 0 | 
-| `label_time` | `int64` | 0 | 
-| `timestamp` | `int64` | 0 |
+All these 5 columns have no `null` value.
+
+| Column | Data Type | 
+|---|---|
+| `user_id` | `int64` |
+| `item_id` | `int64` |
+| `label_type` | `int32` | 
+| `label_time` | `int64` | 
+| `timestamp` | `int64` |
+
+> [!NOTE] **Note:**
+> When `user_int_feats_{fid}` and `user_dense_feats_{fid}` share the same `{fid}`, they are aligned and jointly describe the same entity or signal.
 
 ### User Int Features (46 columns)