Suction cup

The suction cup, also known as the suction probe, (PROSE: Dalek Combat Training Manual) suction pad, sucker pad, (PROSE: Doctor Who in an Exciting Adventure with the Daleks) sucker cup, (PROSE: The Dalek Generation) or simply sucker, was the eponymous tip at the end of the suction arm, the standard Dalek manipulator arm. The Tenth Doctor observed that they were the Daleks' equivalent of a hand. (PROSE: Prisoner of the Daleks)

While a minority of Daleks, such as Elite Guard Daleks of the Time War (TV: The Parting of the Ways) and Vault Daleks of the New Dalek Empire, naturally wielded alternative manipulator arms, (TV: The Stolen Earth) other Daleks could temporarily exchange their suction cups for other appendages which served their needs, such as when Dalek Jast used a syringe to administer a solution for Dalek Sec's Human-Dalek form. (TV: Daleks in Manhattan)

Function
To accomodate the suction cup, 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) As demonstrated by the Bronze Daleks, the suction cups could be used to project fire, functioning as flamethrowers alternative to the pyro-flame or flame gun. (TV: The Dalek Project)

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)

Behind the scenes

 * The suction arm was notably the very first part of a Dalek to appear on-screen, as seen attached to the Dalek which confronts Barbara Wright within the Dalek City at the very end of their first episode, "The Dead Planet". The Daleks would only be revealed in full in their second episode, "The Survivors".
 * In the online video game, The Last Dalek, the player can use the "Metaltron's" extending suction arm as a close-quarter weapon. The suction cup's ability to bypass locks is utilised via a Bio Processor.
 * In Friend from the Future, Bill Potts, within the war zone, is bemused to find that the Daleks have suckers, asking the Twelfth Doctor if they had run out of guns. She then asks if there is "a Dalek out the back with no guns and two suckers that's really hacked off", to which the Doctor answers that he has never asked because he has always been too busy running away.
 * The Power of the Doctor was the first and only appearance of Daleks with standard suction cups in the televised Thirteenth Doctor era following the Reconnaissance Dalek, Defence Drone, Death Squad Daleks and Dalek Executioners, all of which wielded a form of claw manipulator.