How does email tracking work in Portrait Dialogue?

This article explains email tracking in Portrait Dialogue

When you use this email tracking a 1pixel x 1 pixel transparent image is present in the email. 
When this image is requested because an email is opened, a record is created in the WEB_TRACK_LOG table. 
That record links to the CUSTOMER_MESSAGE table which identifies the participant ID, the Message log ID, message bundle log ID, and customer ID.

This is the mechanism that tells you if a customer has opened the email.
This is an industry standard way of tracking emails.
UPDATED:  March 27, 2018