802.1ad. Customer Edge Port vs Provider Edge Port.

So I'm reading 802.1ad, and I've got three definitions in there:

3.6 Customer Edge Port: A C-VLAN component Port on a Provider Edge Bridge that is connected to

customer owned equipment and receives and transmits frames for a single customer.

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3.19 Provider Edge Port: A C-VLAN component Port within a Provider Edge Bridge that connects to a

Customer Network Port and receives and transmits frames for a single customer.

​

3.20 Provider Network Port: An S-VLAN component Port on a Provider Bridge that can transmit and

receive frames for multiple customers.

I understand the difference between the third definition and the first two, but I can't get it what's the difference between the first two.