If an account was using and paying for your product but later left, you would like to mark them as churned in Freshsuccess. An account that is marked as churned was a real account and all historical data associated with that account will be kept. Freshsuccess does not stop event collection for churned accounts per se, but FDCS won't generate the summaries for churned accounts to prevent them from polluting the counts of DAU, MAU, etc.
Churned accounts will show an "Inactive" status in FDCS, which you can use to filter them out in alert rules and reports. Additionally, churned accounts are counted in the total set of accounts and they will show up in churned financials as well.
You can reactivate a churned account at any time you want, you can do so navigating to the account in question > Click the three dots on the top right corner > Mark as Active. Once the account is activated, Freshsuccess will re-process the data generated during the churn period for that account, if there are any.
If you have accounts that were created for testing purposes or they have never become a real customer, you would like to delete those accounts so that their (inaccurate) data do not pollute those of the rest of your accounts. Deleting an account deletes all data associated with that account and removes that account from all throughout the product.
You have the option to re-sync a deleted account from the appropriate 3rd party system by removing that account from the Deleted accounts list under Manage Accounts in Management Center.