According to the Salesforce documentation, the marketer can accomplish the goal of creating different Marketing Cloud Account Engagement lists to correspond with the different stages of the buying cycle and automatically updating the prospect list membership based on the opportunity stage change in Salesforce by using a dynamic list. A dynamic list is a list of prospects that is updated automatically based on certain criteria, such as prospect field values, activities, or scores. A dynamic list can be used to segment prospects based on their opportunity stage in Salesforce, and to add or remove prospects from the list when the opportunity stage changes. For example, a dynamic list can be created for prospects whose opportunity stage is Negotiations, and another dynamic list can be created for prospects whose opportunity stage is Closed Won. When an opportunity moves from Negotiations to Closed Won in Salesforce, the prospect associated with that opportunity will be removed from the Negotiations list and added to the Closed Won list in Marketing Cloud Account Engagement. A completion action, a page action, or an automation rule are not the best tools to accomplish the goal of creating different Marketing Cloud Account Engagement lists to correspond with the different stages of the buying cycle and automatically updating the prospect list membership based on the opportunity stage change in Salesforce, as they are related to other aspects of automation, such as performing actions after a prospect completes a marketing element, visits a web page, or meets certain criteria, but not segmenting prospects based on their opportunity stage in Salesforce. Reference: Salesforce documentation