Bundled Ruleset
SpamAssassin includes a large set of rules, which is optimised at release time to catch the optimal quantities of Ham and Spam mails with the minimum number of FalsePositives and FalseNegatives.
Custom Rulesets
SpamAssassin allows entire third-party rulesets to be installed simply by placing them in the sitewide configuration directory (usually /etc/mail/spamassassin).
...