How is Retention defined in Signal?

The canned Retention events in Signal (2 week, 3 week, 4 week, and 2 month) are “Anything” Retention events. Anything retention means that the user came back and triggered any event in your project in that time frame.

Note: Anything excludes the Message Sent and Message Bounced events that are triggered with Mixpanel Messages

Related article: What does "Anything" include in Retention reports?