The limitations of connecting Marketing Cloud Account Engagement to a Salesforce Sandbox account are: if connected to a sandbox first, when you create your production connector, all prospects will not automatically queue to sync, and Marketing Cloud Account Engagement can't "wipe" data, so you will need to manually delete any prospects and data that is brought into your Marketing Cloud Account Engagement account from the sandbox connector before enabling the connector for your production instance. A Salesforce Sandbox account is a copy of your production environment that you can use for testing and development purposes. Marketing Cloud Account Engagement Sandboxes are test versions of a Marketing Cloud Account Engagement Business Unit that you can provision within a Salesforce Sandbox. However, Marketing Cloud Account Engagement Sandboxes have some limitations that you need to be aware of before using them. One limitation is that if you connect Marketing Cloud Account Engagement to a Salesforce Sandbox first, and then create a production connector, the prospects in your Marketing Cloud Account Engagement account will not automatically sync with the production environment. You will need to manually sync them or use an automation tool to do so. Another limitation is that Marketing Cloud Account Engagement cannot erase the data that is brought into your Marketing Cloud Account Engagement account from the sandbox connector, such as prospects, fields, and assets. You will need to manually delete them before enabling the connector for your production instance, or else you will end up with duplicate or unwanted data in your Marketing Cloud Account Engagement account1112 Reference: 11: Marketing Cloud Account Engagement Sandboxes: What They Can, and Can't Do12: Use Marketing Cloud Account Engagement Sandboxes

The best way to understand how many prospects are currently waiting on a step in an engagement program is to look at the tooltip above that step. The tooltip will show you the number of prospects that are currently on that step, as well as the number of prospects that have completed that step. You can also see the percentage of prospects that have taken a specific action or followed a specific path from that step. The tooltip is a quick and easy way to get an overview of the performance of each step in your program

According to the Salesforce documentation, the two form field options that should be enabled to make the "Comments" field visible and empty every time a prospect returns to one of the forms are: B) Always display even if previously completed, and C) Maintain the initial value upon subsequent form submissions. The Always display even if previously completed option ensures that the field is not hidden or pre-filled with the previous value when the prospect revisits the form. The Maintain the initial value upon subsequent form submissions option ensures that the field is not overwritten with the new value when the prospect submits the form again. These two options combined will make the field appear as visible and empty every time the prospect returns to the form. The Do not prefill option will prevent the field from being pre-filled with the prospect's information from Marketing Cloud Account Engagement or Salesforce, but it will not prevent the field from being pre-filled with the previous value from the same form. The Display other fields in this form based on the value of this field option will create a dependent field that will show or hide other fields based on the value of the "Comments" field, but it will not affect the visibility or emptiness of the "Comments" field itself. Reference: Salesforce documentation

Explanation:
Automation rules and segmentation rules are both types of Marketing Cloud Account Engagement automation that allow you to perform actions based on criteria. However, the main difference between them is that automation rules run continuously and segmentation rules run once. Automation rules are always on and listening for changes or activity, and they can apply actions to prospects that match the criteria now or in the future. Segmentation rules are designed to pull a one-time list of prospects, then apply the set actions, such as adding them to a list or a Salesforce campaign. Once run, segmentation rules have to be copied to run again

The event that would make the Salesforce connector become unverified is the connector user's Salesforce password was changed. A Salesforce connector is a feature that allows you to connect your Marketing Cloud Account Engagement account with your Salesforce CRM system, and enable bi-directional data sync and alignment between sales and marketing. To verify the Salesforce connector, you need to have a connector user, which is a Salesforce user that Marketing Cloud Account Engagement uses to access your Salesforce data and perform actions on your behalf. The connector user needs to have the appropriate permissions and settings in Salesforce, such as the API Enabled and Modify All Data permissions, and the security token2. If the connector user's Salesforce password was changed, the security token would also change, and Marketing Cloud Account Engagement would not be able to authenticate with Salesforce. This would cause the Salesforce connector to become unverified, and the data sync between Marketing Cloud Account Engagement and Salesforce would stop. To fix this, you need to update the connector user's password and security token in Marketing Cloud Account Engagement, and re-verify the Salesforce connector