I am not sure what do the reports represent. Can you explain?

Passes registered in wallet - This report shows the number of all passes saved in a device at any give time. You can use it to view the trends - is the number stagnant, growing, or is it decreasing? If the number of passes removed from a device is higher than the number of passes newly downloded, you will see the graph going down. If that happens, we recommend looking into what might be causing the decrease. It could be caused by factors like too many push notifications, not enough added value for the end-user etc.

Passes at least once registered in wallet - This report shows the total number of passes that have been ever downloaded into a device, not taking into account whether the pass has been later removed from the device or not. This number tells you the maximum reach you had, the total number of people that completed the download. You should be looking at both the number of passes registered in wallet and passes at least once registered in wallet to find out the retention rate (the more similar these two graphs look the better the retention).

Downloads of pass by tags - This graph tells you how many people downloaded a pass with a certain tag. Let's say you decide to share a pass on Facebook, on a news site and via email. You could tag create links that will be tagged facebook, news and email and then observe how effective each channel is.

Previews of pass by tags - This graph is very similar to Downloads of pass by tags, but this one tells you have many people clicked on the link and viewed the preview page. That makes for a very useful comparison, because again you are able to see the maximum potential reach of everyone who was interested enough to click on the link and compare against the number of people who actually completed the download process. Again you want both statistics to look as similar as possible. If the number of previews was disproportionately higher than the number of downloads, you might want to think about the appeal the pass has for your customers. If the preview page does not convince them to finish the download, the information shown on the pass might not be attractive enough for the customer.