Extra Invisalign tray? What does this mean and how should I proceed? Any help would be appreciated, thanks!

Hello! Each of my Invisalign trays has been the usual 1 of 26, 2 of 26, 3 of 26 - up until the current 14 of 26 - for both Upper and Lower. However, I have just noticed an extra set *between* 14 and 15 containing a single lower tray inside, with the same number 15 as the next set. It also has an extra label of "Attachment Template." What does this mean and how should I proceed? Any help would be appreciated, thanks!