ENDIAN Firewall - Destination NAT Questions

I have two questions about the Destination NAT configuration screen and I am hoping that some veterans can help out a newbie.

1. What does the NAT selection do?
I understand the functionality of NAT, but I found that it doesn't seem to make any difference on my network if I activate it or not. For example I have an internal server running on 192.168.x.A. From what I can see I am fully connected no matter if I direct my static IP to it using NAT or without. Is disabling NAT on this configuration screen an equivalent to a 1 on 1 NAT setup in other firewalls? I mean if I disable NAT and it really wouldn't do any NAT I would otherwise expect my connection to fail...

2. I setup 5 additional static IPs (IP_1 to IP_5) on my PPPoE setup screen. Now, I noticed that the modem itself uses another IP Address (IP_6) as the default address. Unfortunately, that address does not show up in the configuration of Destination NAT. It only shows me the 5 additional IPs. How can I route traffic from that default address? Should I just specify IP_6 under additional addresses in the PPPoE setup as well?

Thanks,
Jens