All links in your Email Marketing campaigns are momentarily redirected via servers to capture clicks. So when a contact clicks on a link in your email, this redirect helps us track which contact clicked what link. For more details, read our guide to viewing clicked statistics.
Social shares are tracked in a similar way. Unique social share links are generated for each recipient in your contact list. So when the share link is clicked, you'll see exactly which contact did the sharing.
If a reader received your email successfully, but took no action, this contact is tracked in your Accepted stat.