When the "Send as Transaction Email" box is checked in the tactic builder, the system will ignore any unsubscribes and send the transactional email to any contact that falls in the audience segment based on chosen segment criteria. Examples of transaction emails are invoices, order confirmations, shipping notifications, etc.
Use this feature at your discretion. Review the laws and regulations under the FTC CAN-SPAM Act for more information.
For more information review this transactional email article in Business News Daily.