pf.conf syntax

[Scott Gerhardt <scott_(_at_)_g-it_(_dot_)_ca>]

Question regarding pf.conf syntax:

In the default pf.conf has a nat entry as follows:
nat on $ext_if from !($ext_if) -> ($ext_if:0)

The rule is self explanatory, but I don't understand the significance 
of the ":0".  I grep'd all the docs, googled, and searched the 
documentation but I can't find any explanation of what the ":0" 
represents.

What does ":0" appended to the "$ext_if"  macro represent?



Thanks,

--
Scott A. Gerhardt, P.Geo.
Gerhardt Information Technologies