For the last three+ years I have been sending out tracked emails in protest of corruption at the California State Bar. Each time the email is opened on a different computer/cell phone, the count increases.
In Example 10 below,
1 open means Maureen McCarty opened the email.
3 opens usually means Assembly member Gershenzon opened the email and then forwarded it to someone, and they replied back. Or she forwarded it to two people.
4 or more indicate a conversation.