OK, I see what you mean.
Backplane here means the dedicated VCP cables.
On EX4500, you can have 2 VCP. Each VCP cable has 64Gbps full duplex. Two cables, 128Gbps full duplex.
On EX4550, you have a slot where you can add 2 more VCP cables. Hence the 256Gbps full duplex
Dataplane rate is for network ports. And in ex4500 you can have up to 48 10G ports. This is 480Gbps simplex, 960Gbps full duplex. That's why it's unblocking. The data plane rate matches the maximum possible traffic that can be sent through the ports.
The number of maximum pps comes probably from the chipset used in the platform.
In case you need to find out how many packets you need for wire speed:
http://kb.juniper.net/InfoCenter/index?page=content&id=KB14737
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