When you look at a message, you see things that indicate the message is spam or not, what words it uses, how well it was written, where it came from, etc.
So SpamAssassin reads your mail and assigns a weighted score based on what it sees that indicate the message might be spam.
So after the message has been scored, you can do three things: move it to the Inbox, tag it as spam and then move it to the Inbox, and finally stick it somewhere else besides the Inbox ("quarantine").
So you get to choose how high a score is needed to "tag" a message and how high the score needs to be to quarantine it instead. And the quarantine means that most of the spam messages will be grouped together instead of mixed in with your legitimate messages. There is no hard number for the scores; if too many of your legitimate messages are being tag or quarantined, lower the score. See Junk Mail Settings
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