VPLS requirements question

Hi,
 
I have the following questions:
 
1) Section 3.5: What does this sentence mean? "A virtual layer 2 forwarding entity that 
   is closed to a VPLS domain membership"
 
2) Section 4: Is the definition of WAN link correct (ie., a link between CE and PE)? and if so why is it complex?
 
3) Section 9.7 is not clear. Is service provider injecting L3 traffic? and if so in which
direction? what is the purpose and what is the format of those packets?
 
Thanks,
-Shahram