People often like to be notified that they have received new messages, but they don’t want to hear about new spam messages (those should be ignored), only good ones.
This option makes SpamSieve show the number of new good messages in its Dock icon. “New” means “since the last time the number was reset”; it’s not related to the number of messages that are unread. “Good messages” means “messages for which there is a Predicted: Good entry in SpamSieve’s Log window.” If the number is unexpectedly high, you can look in the log to see which messages SpamSieve processed recently. Perhaps it was asked to reprocess old messages or to process the same message multiple times.
If there are no new good messages or if SpamSieve or the mail program is frontmost, SpamSieve will not show any number (rather than showing zero).
Clicking the Dock icon, activating your e-mail program, or training SpamSieve with a message will reset the count.
If you’re using DragThing 5.3 or later, SpamSieve can show the message count in its tile in DragThing’s process dock.