It is good practice to mark an issue as complete when it has gone to the printers.
Issues are marked as complete by going to the Publications screen and toggling the "Complete" switch in the column on the right to on (it shows green).
How it affects artwork
When an issue has been marked as complete, it is no longer possible to change the artwork or the artwork status for any bookings in this issue. The Upload Artwork and Clear Artwork buttons will be greyed out. This prevents you deleting artwork used in a previous issues. (If you need to do this for any reason, you can temporarily mark the issue as not complete while you make your changes).
Otherwise, to add new artwork you need to go to the booking and add a new advert to the booking line for that issue and upload the new artwork to the new artwork box.
How it affects the client bookings tab
On the client Bookings/Advets tab - when an issue has been marked as complete the booking will appear greyed out see July 2020 below.
How it affects the main clients screen
Active Tab - Only clients with a booking in the current or future issues will appear on the active tab.
Inactive Tab - Clients who have previously booked but have no current or future bookings will appear on the inactive tab - provided all previous issues they were booked into have been marked as complete.