My guess is it is an anti-tamper sticker. Once applied to a surface, removing it leaves behind a decal so that it would be obvious should the sticker be reapplied.
Best guess is that it is supposed to have been used to make sure remains are not mixed up. Black sticker with the paperwork and black sticker with the remains with black sticker with the urn sort of thing.
Best answer would come from the burners, call them and find out.