What is the difference between creating an alert for a specific account and leaving it on All accounts?

With a specific account selected, the alert evaluates only that account's metrics, and the notification includes the account name, so it's easy to tell which account it's about. With All accounts, the alert evaluates the combined data of all the accounts you have selected in the dashboard: spend, clicks and impressions are added up, and cost and return metrics are calculated from those totals. This means a high CPA on a small account can get diluted in the total. If you want to monitor each account closely, create one alert per account. You need a connected Meta account and accounts selected in the dashboard for alerts to work.