Labels used as Promotions/Advertisements on windows are commonly known as window stickers as they are mostly designed to be stuck on the inside of the window for people to read on the other side. There are a few ways this can be achieved and printing on the adhesive side is certainly possible. However, this can be an expensive process and there are other ways to achieve this which are more cost effective.
When printing onto clear material it is possible to mirror the print, which simply means that the text can be read correctly from the other side of the clear label. When printing onto clear material, sometimes the print/colour can appear washed out or transparent. In this case, white ink can be added on top so that the mirrored text is still readable from the other side. The result is that the full colour (CMYK) print is more vibrant.
When referring to window stickers or labels, this could also mean the labels that glass & window manufactures use. Glass Removable or Ultra-Removable is the most commonly used for glass as when removed it won’t leave any obvious residue like a standard removable might. See more about the adhesive required for these kind of labels here.