meaning of "both" in a filter statement

[Hal <hal () mrj com>]

This is soemthing of a newbie question but I wonder if anyone can set me straight  on it:

Many firewalls have rules in the form  (Action, interface, source, source-port, destination dest-port)
where action is the usual permit/deny, interface is outside or inside, source, destination are what they say and permit 
wild cards (subnets). OK. 

My question is this.  On some firewalls the interface spec also includes (besides terms for inside, outside, 3rd) a
term "both." That means apply the permit/deny on traffic appearing at both inside (trusted) and outside (internet) 
interfaces. 

This at first glance seems absurd.  It means that traffic going to D from S can move in either direction across the FW. 
A very unusual  arrangement  with almost no uses.   Obviously there must be a more reasonable explanation 

Has anyone found an explanation for what "both" really does.?


Regards Hal
hal () mrj com