BoltMail
"Open" and "Unique Open"
"Open" is the number of times your email campaign has been recorded open,
an email can be opened multiple times, and it will be recorded multiple times.

While "Unique Open" only recorded once for each email.

Example:
Contact A open his email 3 times,
Contact B open her email 2 times,
Contact C open her email 5 times.

Open = 10
Unique Open = 3