There is an example of round-robin weights in pf.conf(5):
Code:
In this example, a NAT gateway is set up to translate internal addresses
using a pool of public addresses (192.0.2.16/28). A given source address
is always translated to the same pool address by using the source-hash
keyword. The gateway also translates incoming web server connections to
a group of web servers on the internal network.
match out on $ext_if inet from any to any nat-to 192.0.2.16/28 \
source-hash
match in on $ext_if proto tcp from any to any port 80 \
rdr-to { 10.1.2.155 weight 2, 10.1.2.160 weight 1, \
10.1.2.161 weight 8 } round-robin
The GRAMMAR section shows
weight defined as a component of a
host.