Base unit

The base unit was the lower half of a Dalek casing, located immediately below the weapons platform, which was studded by sense globes. (PROSE: Prisoner of the Daleks) It served as a Daleks' means of mobility and ascension to hover position. (TV: Dalek)

Davros' chair typically appeared as a black base unit with silver sense globes. (TV: Genesis of the Daleks et al.)

Characteristics
Should a Dalek chose to open their casing, the front portion of the base unit could split its panels apart. Though this was not always executed, (TV: Dalek) it proved necessary for Dalek Sec to do so during the Final Experiment, as it allowed his new Human-Dalek form to walk out of the casing. (TV: Daleks in Manhattan) Likewise, a Dalek's casing was completely opened to accomodate Clara Oswald. (TV: The Witch's Familiar)

The Earthmen Detector was equipped onto the casing through a band encircling the base unit. (COMIC: The Daleks Are Foiled)

Sense globes
Sense globes were evidently the instruments of the Dalek self-destruct process; the Dalek named Metaltron ejected all its sense globes to create a field in which it was completely destroyed. (TV: Dalek)

One Dalek, a recon scout, built itself a new casing in 2019 Yorkshire which had short-range missiles behind each globe. One of these missiles made short work of a tank which sought to destroy the Dalek. (TV: Resolution)

As a distinguishing feature
As well as a black dome, the Dalek saucer commander was distinguished by alternating black-and-silver panels on the base unit. (TV: The Dalek Invasion of Earth)

Individual cases
Footage of a Dalek's base unit and sense globes was captured by a British news broadcast during the Battle of Canary Wharf in 2007. (WC: Tardisode 13)

To complete the Empire State Building before the gamma radiation lightning strike hit Earth in 1930, three of the panels with sensor globes of Thay's base unit were removed and attached to the building's mast, as the Dalekanium of the plates was required to attract the gamma strike. In their place, Thay was given crude iron replacement plates. The Tenth Doctor attempted to remove them too late as the lightning struck him, resulting in Dalek-Humans infused with Dalek Sec's altered DNA as well as the Doctor's Time Lord DNA. (TV: Daleks in Manhattan/Evolution of the Daleks)

Behind the scenes

 * In the video game The Last Dalek, based on the events of Dalek, the Metaltron gains the ability to ascend over stairs and manholes only after acquiring an anti-gravity device. It is further able to accelerate via the Speed Boost.