After publishing an issue of your publication you will see summary stats below the issue header, something like this:
is the percentage of registered subscribers who opened the issue at least once. The number of subscribers is recorded at the time of publishing and used to calculate the percentage, this means that any new subscribers joining after the publishing date will not cause the percentage to become lower.
is the percentage of registered subscribers who clicked at least one link anywhere in the issue. Again, new subscribers that join since the issue was published will not affect this number.
is the total number of clicks on any link in an issue, this includes clicks that were not able to be attributed to a specific subscriber (for example, clicks from the web version of the issue).
is the total number of unique clicks on any link in an issue, however multiple clicks by the same subscriber will not be counted twice.