NAT & (interface) = round-robin between IPv4/IPv6 addresses?

[Ed White]

Happy new year everybody,

I have a quick question. I am using OpenBSD 4.2-stable.

I noticed that with the following NAT rule:
nat on sis1 from 10.2.2.0/28 to any -> (sis1) static-port

I get the following output:
# pfctl -sn
nat on sis1 inet from 10.2.2.0/28 to any -> (sis1) round-robin static-port

This is the interface:
sis1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        lladdr xx:xx:xx:xx:xx:xx
        groups: egress
        media: Ethernet autoselect (100baseTX full-duplex)
        status: active
        inet6 xxxx::xxx:xxxx:xxxx:xxx%sis1 prefixlen 64 scopeid 0x2
        inet zz.zz.zz.zz netmask 0xffffff00 broadcast zz.zz.zz.zzz


My question is simple: is that "round-robin" actually used?
If it really means that PF sees 2 or more IPs, what are these IPs?

Thanks.