The labels are those shown on the Labels 3 x 4 layout - Thumbnail Label 1, etc. In turn, they are calculated from values in the Thumbnail Label Selections field. This is the field with checkboxes on the Thumbnails layout.
To make changes, you would first need to update the Thumbnail Label Choices value list. Then you may need to adjust the position of the buttons on top of the checkboxes.
It's not easy and there is probably a better way to do it in the first place. The templates are not perfect ;)