I am playing with openbsd PF, and i read the text below:
(http://www.countersiege.com/doc/pfsync-carp/)

"When writing the rest of the pf ruleset, it is important to keep in
mind that from pf's perspective, all traffic comes from the physical
interface, even if it is routed through the carp address. However, the
address is of course associated with the carp interface. Therefore, in
the interface context, such as "pass in on $extif ...", $extif would
be the physical interface, but in the context of "from $foo" or "to
$foo", the carp interface should be used, as it's being meant in the
address context."

Why the carp "interface" cannot be used in context of the interface?

Thanks for your time and cooperation.