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Discs were sported at the back of the middle section of the casing of all Daleks who operated on Earth during the 22nd century Dalek invasion, designated the Dalek Earthforce, from the basic Daleks to the Dalek saucer commander to the Supreme Controller.
Upon encountering these Daleks, Ian Chesterton and the First Doctor, who had previously encountered the Dalek War Machines on Skaro, acknowledged their difference in appearance. Recalling that the Daleks they had met could only move on metal within the Dalek City, Ian suggested that the discs accounted for their increased mobility on Earth. Indeed, during the occupation, the Daleks were able to traverse various forms of terrain. (TV: The Dalek Invasion of Earth, PROSE: Doctor Who and the Dalek Invasion of Earth)
Later Daleks lacked the discs while retaining their increased mobility, instead sporting a series of slats across the middle section. (TV: The Chase et al.)
Plasti-Discs were required to enter the Braxiatel Collection. (PROSE: Professor Bernice Summerfield and the Doomsday Manuscript)
Behind the scenes
The discs were absent in the theatrical adaptation of The Dalek Invasion of Earth, Daleks' Invasion Earth 2150 A.D..