The dastar exists because of kesh, uncut hair, which is the first of the five articles of faith of the Khalsa. The hair is not to be cut, so it must be kept, and the turban keeps it. A second of the five, the kanga, a small wooden comb, is worn in the hair under the turban, so two of the five Ks are involved in the same act of dressing.
It is tied fresh each day from a length of cloth, commonly five to seven metres, and that daily tying is part of why it is not treated as a hat. A hat is put on; a dastar is made. It is not removed on entering a building or in the presence of authority, and requests to remove it are experienced as requests to be uncovered rather than as matters of etiquette.
Styles carry information. The rounded dumalla is worn in several forms, and the Nihang order wears a large one, sometimes with a chakram, a steel quoit, and other insignia set into it. The pointed Delhi or Nok style is common among Punjabi Sikhs. The Patiala shahi is broad and layered. The keski, a smaller under turban, is worn on its own by many Amritdhari women and by children, and by men under a larger turban.
Colour is largely personal, though certain associations exist: deep blue and saffron with the Khalsa and with Nihang tradition, white with older men and with mourning in some families.
Sikh women may wear a dastar and a significant number do, particularly among the initiated, while others cover with a chunni. The choice is treated within the tradition as open, and the reasoning offered on each side is about equality of obligation rather than about custom.