To Humans, the "TAG" is a tracking device to tell where bats migrate to. Another form of "band", as it was called by Shade, was used by the humans in a war (Sunwing). These bands, unlike the ones places on the arms, were tied to the bat's stomachs, and a microchip was placed on their ear. The microchip "sang" to the bats and owls, telling them to land on certain buildings. When they landed, the bands exploded.
Bats consider them to be "BANDS." Goth and Throbb believe the bands are a bad thing, but no bat knows what it is used for. Marina and Frieda have bands of silver. Goth and Throbb have bands of black metal.