definition of firewall(FW) NIC interface

[tfirkins]

When configuring firewall interfaces, three choices are offered, trusted(int), untrusted(ext) and firewall(fw). What is the purpose of firewall (fw)? My guess is that it serves as a single secure channel for webtool access, as opposed to just accessing it from the trusted interface, where potential hackers might want to get in and change rules. Just want to be sure.