From Wikipedia, and as I remember it from my days growing up in a Radio/TV repair shop....
"Tubes intended for television use normally have an aluminised interior which acts as a reflector on the back of the screen, and forms one plate of a capacitor. Aquadag is also painted on part of the outside of many tubes. The glass, tube, internal aluminium coating and the outer Aquadag then form a capacitor used to filter the high voltage EHT supply."
As I recall, painfully, failing to properly position the ground clip against the AquaDAG could result in a painful staic discharge. ...Jim Thompson