The difference between UserId and InitiatingUserId in Plugin of Dynamic CRM 2011

I would like to discuss the difference between UserId and InitiatingUserId of IExecutionContext interface in Dynamic CRM 2011 plugin. The explanations of them are given in MSDN like this:
• IExecutionContext.UserId Property: Gets the global unique identifier of the system user for whom the plug-in invokes Web service methods on behalf of. ^[1]
• IExecutionContext.InitiatingUserId: Gets the global unique identifier of the system user account under which the current pipeline is executing. ^[2]
in Simple:

The context gives us both the GUIDs

• context.initiatinguserid: gets the systemuser GUID who actually fired the plugin
• context.userid : gets the  impersonated systemuser GUID

From the official definition, I feel it is still not very clear. So I set up an experiment in my development environment.

1. Create a Test Entity record and assign it to another user in system:

 

2. Add a plugin in the update stage of Test Entity

 

3. In this plugin, the code snippets of core part are:

 

 

This plugin is running as a system user and assigns ownership of the Test Entity to the IExecutionContext.UserId.

If we look at the Test Entity it is now assigned to someone else (on form refresh):

 

4. Now, uncomment 2nd line and comment out the 1st line so we use IExecutionContext.InitiatingUserId

5. Run plugin again and it will change the ownership:

 

Now we can see the difference between the two options

• With “UserId” the plugin is running under system context, and the “UserId” gives the ID of user we define in plugin, namely “System”. It focuses on “behalf of”.

• Whereas the “InitiatingUserId” is the ID of normal CRM user whose action triggers plugin at the first place.