Mailable prospects are prospects who have opted in to receive email communication from you and have not bounced or unsubscribed. You can find the number of mailable prospects in your database by navigating to the prospect list and selecting "Mailable Prospects" from the dropdown menu. You can also filter the list by tags, campaigns, or custom fields to segment your mailable prospects. Reference: [Mailable Prospects], [Filter Prospects]

A segmentation rule is a type of rule that allows marketers to segment their prospects based on specific criteria, such as field values, activities, or scores. A segmentation rule runs only once, as soon as the user completes the rule creation process. The rule will not run again, unless the user manually re-runs it or schedules it to run at a later date. A segmentation rule does not run continuously, up to five times, or as many times as the user specifies when they create the rule1. Reference: 1: Segmentation Rules

According to the [Salesforce documentation], the best place to look for all automation rules and engagement studio programs that a prospect has been a member of is the Prospect's Audits tab. This tab shows a detailed history of all the actions and activities that have occurred for a prospect, including the automation rules and engagement studio programs that they have been added to, removed from, or completed. The Automation settings, Scoring rules, and Lifecycle report do not provide this information, as they are related to the configuration, scoring, and reporting of the automation and engagement features, not the individual prospect's history. Reference: [Salesforce documentation]