There are many quantities in the electronic structure community in determining the character of an excitation, such as natural transition orbitals, 199-201 transition density, difference density, electrostatic surfaces/charges,, 202,203 etc. In principle, they can all be generalized for polaritons and be able to used to characterize the nature of polaritons. Our main focus here is on the transition density, which is the most straightforward quantity that can be obtain from electronic structure packages. The one-electron polaritonic transition density between the ground and a th polaritonic states can be written as