Listing in SpamCannibal's results from sending spam or virus ladened email to their mail-servers (spamtraps). SpamCannibal can be used to block specific IP Addresses or entire ranges of IP Addresses; this also means your IP Address can be listed if you are sending messages with BOGUS/FORGED headers, i.e. you don't run a mail-server and have a generic dynamic address and connect to a hosted mail-provider. Or, if you operate a server, you do not have the proper Reverse-DNS entry in place with your ISP.
Spamcannibal Reports Subnets
Subnet-based Blacklists are used to reject email from entire ranges of IP Addresses, i.e. providers that are hosting companies sending spam, as well as single IP Addresses that may fall in that range of IP Address.