The Threat Rating
The Threat Rating is a metric that describes how dangerous an IP is based off its observed suspicious activity. These activities include sending spam messages, performing dictionary attacks, harvesting addresses, posting spam comments to web forms, hosting bad web pages (phish sites, etc), and breaking nofollow or certain robot.txt rules.
A Logarithmic Score
The Threat Rating is a logarithmic score -- much like the Richter's scale for measuring earthquakes. A Threat Rating of 25 can be interpreted as the equivalent of sending 100 spam messages to a honey pot trap.
|IP that is as threatening as one that has sent
|100 spam messages
|10,000 spam messages
|1,000,000 spam messages
As seen in HTTP:BL
The Threat Rating is the third octet returned by a result IP from HTTP:BL.