When the account has a Pixel, Ads Editor looks up the events it has fired recently, including standard events (Purchase, Lead, AddToCart, InitiateCheckout and others) and custom events created by you or by tools on your site. The green number in parentheses shows how many times the event fired in the period analyzed, and these events come selected. Events with a dash are standard Meta events with no recent activity. They appear so you can still create the audience, but they come unselected. Each selected event creates one audience per retention period, named in the pattern SITE - [EventName] - 30D. If an event name is invalid for Meta, that specific item shows Error in the progress list, without affecting the others.