Suction cup

The suction cup was the eponymous tip at the end of the Dalek suction arm. (PROSE: Prisoner of the Daleks) For this reason, Dalek technology often had circular controls which the Daleks could arm or disarm by rotating. The black tip at the end, which gave the arm its "plunger" shape, could alter its form to better interact with specific objects, for example, finding the combination of electronically-locked doors. (TV: Dalek)

Even if the Dalek died, its arm could still operate machinery for a live person. (TV: Remembrance of the Daleks) The arm also had a telescopic feature to extend its length. (TV: The Dalek Invasion of Earth, Dalek)

It could also serve as a weapon; with the tip placed over the nose and mouth, the arm could generate suction to suffocate and kill a human and enough traction to crush a human skull or rip through sheet metal. (TV: Dalek, TV: Doomsday, PROSE: I Am a Dalek) The Tenth Doctor claimed to have seen Daleks crush bricks with ease. (PROSE: Prisoners of the Daleks)

At least in the case of the Cult of Skaro, the arm could also scan brainwaves for information or act as a quick intelligence scan. (TV: Doomsday, Daleks in Manhattan) However, if a large amount of information needed to be analysed, it could be drained out by force by encasing a subject's head inside their "plunger" tips, which would stretch the victim's head and brain to wring out their knowledge through information probes. The probes were strong enough to have a deadly effect on humans, draining them so fiercely that their bodies shrivelled up and became burned out husks. (TV: Doomsday)

Imperial Daleks had a more conical variant, with a slotted rim specifically designed to operate controls. (TV: Remembrance of the Daleks)