What is the difference between "Node Type Bound" and "Hierarchy Set Bound" ?

Dear everyone,

I am familiarizing myself with the EDMCS and have something I donot understand.

When I tried to:

Step 1: Create Node Type

Step 2: Create a Hierarchy Set based on the node type of step 1

Step 3: Create a Node Set based on the hierarchy set of step 2.

However, the bound status of the Node Set that I created in Step 3 is "Node Type Bound"

I understand that "Node Type Bound" only exports members' properties to the target application and cannot create new members.

But my question is:

What is the system's engine for categorizing the bound status as "Node Type Bound"? Why isn't it "Node Set Bound" ?