While a marketing email can consist of any type of promotion or any other email communication and customers can unsubscribe from receiving these, transactional emails are sent to support a specific transaction between the sender and the recipient and will be delivered to the recipient regardless of whether the customer has unsubscribed from Pathfinder marketing emails or not.
This means if a customer is unsubscribed from marketing emails in Pathfinder, they will not receive any other marketing email from you, until they opt-back in. But, if an email is marked as transactional it will be delivered even to the unsubscribed contacts.
Why transactional emails?
While marketing emails are mostly sent in bulk to a larger contact group subscribed in Pathfinder, transactional emails can be sent to one person as a result of an action triggered by the user (even if unsubscribed). Most often, transactional emails include:
"Thank you for purchase / [or other action]"
Email address confirmations
Password resets etc.
IMPORTANT: As transactional emails do not contain %UNSUBSCRIBE% links, they must not be used to send marketing / promotional content and shouldn't be sent in bulk as they might be reported as spam by the end recipients, which will affect your email deliverability and sender reputation.
Further read: How to create transactional emails?