Pending - normally occurs when we're adding prospects to Step 1. This happens when:
We have a sending window set up. If we're adding prospects the same day that the sending window allows us to send emails, they will enter step 1 immediately. However, if we're adding prospects on a day that's not supported by the sending window, the prospects will be on 'Pending' and will only enter the Step 1 on the next available day based on the sending window.
If the number of prospects we're adding is greater than the number set up in the Daily Prospect Start. Ex: If in the campaign we set up 15 as our Daily Prospect Start, then we're adding a total of 20 prospects. What's going to happen is that the 15 prospects will enter Step 1, but the remaining 5 will be on pending and will enter the campaign(Step 1) on the next day.
If we schedule the prospect to start in the future date, the prospects will remain on pending and will enter Step 1 on the day they're scheduled to go out.
Upcoming - means that the emails are not due yet. We commonly see this when a prospect moves to the Next Step. Ex: If we send an email to a prospect in Step 1 and our next email/step will send in 3 days(if no reply), then the prospect will be on "Upcoming" status until it hits the 3rd day.
Scheduled - means that the emails have been processed and are ready to send on the scheduled date. We usually see this in auto-send campaigns. However, this can also occur in manual send campaigns. Ex: The sending window setup is 8 AM - 5 PM. However, we pushed the "Send All' button on the campaign at 8 PM. The system will start processing and will try to send the emails, but once it detects the sending windows and realizes that it can't send emails at 8 PM, then it will schedule the emails to go out tomorrow starting at 8 AM (following the sending window).
Due - in the manual campaign setup means that the emails are ready to be sent. However, the emails will not send until you click the "review email" and the "send now" buttons to trigger the sending.
Processing - we usually see this when we're trying to send emails. Once it shows that the emails are processing, that means the emails are in the process of being sent. If the sending window allows us to send emails, then the emails will be sent; however, if we're trying to send outside the sending window, the emails will be processed then will be scheduled based on the sending window.
We may also notice 'processing' on the First Step of the campaign and if we have the auto-send enabled. By default, there's a 15-minute delay before the emails start to send out when auto-send is on. This is to make sure that users would have enough time to make changes to the campaign or perhaps remove a prospect that may have been mistakenly added to an 'auto-send' campaign.