When you go to look at the campaign you have just sent in reports you may notice that there is two types of opens; opens and unique opens. There is a slight difference between these.
An open is the number of times the email has been opened.
A unique open is how many people have opened your email.
So the difference is you could have 200 opens on an email but only 10 people have opened it 10 times. Or you could have 200 opens and 200 people had opened it.
Contact A opens the email 5 times.
Contact B opens the email 3 times
Contact C opens the email 10 times.
When you look at the reports, it would say 18 opens but there would only be 3 unique opens.