Update 4.schema.md (#1809)

docs-2.0/3.ngql-guide/6.functions-and-expressions/4.schema.md

@@ -114,6 +114,10 @@ nebula> GO FROM "player100" OVER follow \
 +-------------+-------------+
 ```
 
+!!! note
+
+    The semantics of the query for the starting vertex with src(edge) and [properties(`$^`)](../5.operators/5.property-reference.md) are different. src(edge) indicates the starting vertex ID of the edge in the graph database, while properties(`$^`) indicates the data of the starting vertex where you start to expand the graph, such as the data of the starting vertex `player100` in the above GO statement. 
+
 ### dst(edge)
 
 dst(edge) returns the destination vertex ID of an edge.
@@ -135,6 +139,10 @@ nebula> GO FROM "player100" OVER follow \
 +-------------+-------------+
 ```
 
+!!! note
+
+    dst(edge) indicates the destination vertex ID of the edge in the graph database.
+
 ### rank(edge)
 
 rank(edge) returns the rank value of an edge.