some detailed explaination of the Scope properties

doc/design/scope.md

@@ -9,16 +9,30 @@ Scope is an important concept in programming languages, which defines a program
 
 A detailed explanation of these two attributes goes as following.
 
+
 ## Scope is a Container of Variables.
 
-    * Scope contains Variables as it's data member.
-    * Scope contains methods that are used to manage Variables, such as Create/Get/Delete.
-    * every variable only belong to one certain Scope.
-    * Scope should destruct all Variables within it when itself is destructed.
-    * Variable can only be created by Scope.
-    * Variable can only be got from Scope.
+Scope is used to provide a running environment for Net.
+
+1. Scope mainly has Variables as it's data member.
+    Scope is a running environment for Net. Net should get all it need to do computation from a scope, such as data buffer, state(momentum) etc.
+    All these data/state can be abstracted and create as variable in Paddle, so the only thing Scope need to care about is Variable.
+1. Variable can only be created by Scope.
+1. Variable can only be got from Scope.
+1. Scope contains methods that are used to manage Variables, such as Create/Get/Delete.
+    Because we only need to care about Variable, we only need method to manage the lifecycle of Variable.
+    - `Create` is used to create a Variable by its name and add the mapping relation.
+    - `Get` is used to find a Variable by name.
+    - `Delete` is used to remove a Variable because sometimes we want to release memory or other resources.
+
+1. Every variable only belongs to one certain Scope.
+    Variable can not be shared between nets, if we want to use variables from different scope we can use `Parent scope`.
+
+1. Scope should destruct all Variables within it when itself is destructed.
+    Because Variable can only be got from Scope, when destroying Scope, we also need to destroy all the Vars in it.
 
-    * Scope do not contains Operators and have no information to run them.
+1. Scope do not contain Operators and have no information to run them.
+    Net is designed to drive the computation, Scope is only used to provide a running environment.
 
 ```cpp
 class Scope {