How to read this table for RB951G-2HnD? What is the max throughput?

That will depend on a lot of factors, like firewall rules, kind of traffic, etc, that's the reason of that table with different packet sizes and config conditions, to provide an idea of raw power.

What will be its duty? Internet bandwidth? Size of the network behind it?

That will depend on a lot of factors, like firewall rules, kind of traffic, etc, that's the reason of that table with different packet sizes and config conditions, to provide an idea of raw power.

What will be its duty? Internet bandwidth? Size of the network behind it?

Which column of byte is the best reference? 512k, 64k table?

64k is the best for gauging of raw CPU power, but again, it depends on the chores the router is carrying away, if you'll be using fasttrack or not, etc.

For real life application, you can look at your current stats. For example my WAN counter says total RX 129GB over 199 million packets. That means average packet is about 0.6kB. Obviously, this WILL vary a lot, based on content, which users access. Games usually have smaller packets while downloads go for larger packets. Both are equally important.

However you asked for "max throughput" (without any condition). There is only one such thing and that's the highest number written in the table. Anything else is "max throughput under certain conditions" .
Without specifying those conditions, the number can be anything.

All of you, Thank you!