How transparent is "transparent"

[RichardBenfatto]

Confused in reading documents regarding transparent and virtual wire.

I have a unit in NAT mode but some customers do not want to change network configuration and still have protection. Fine, this is when transparent mode comes in.

So, what is virtual wire? Is it needed or not and when?

Example: Have a router doing the NAT and firewalling with port forwarding to internal Web and mail server. So, unit in transparent mode, policy to allow internal to out and this is it?

My confusion is the virtual wire............say that in the example just described the edge router/NAT.Fireall to the internet ALSO does the DNS and the DHCP for the internal network, as the fortigate is now in the middle, is that when the virtual wires is needed? This virtual wire has nothing to do with the Web ane email services behind the NAT (of the edge router) ? There is no "port forwarding"? Well Fortigate is now a layer 2 only.

Found two different diagrams in Cook Books but it is confusing or incomplete the way it is put in there.

Any ideas?