A habit is not put on at will. It is conferred, in stages that track a formation lasting years, and each stage adds something.
A postulant, in the first period of trial, generally wears ordinary clothes or a partial habit. Clothing or investiture admits the candidate to the novitiate and confers the habit itself, traditionally with a white veil for women to mark that the person is a novice. First profession, the taking of temporary vows, changes the veil to black in many congregations or adds an element such as a cincture, a ring or a cross. Solemn or final profession completes it. In some orders a new name is given at clothing, marking the change of identity the garment represents.
The pieces carry meanings that are stated in the rites. The tunic is the basic garment. The cincture, a leather belt or a knotted cord, refers to chastity and to service; the Franciscan cord has three knots for poverty, chastity and obedience. The scapular, hanging front and back, is called in the Rule of Saint Benedict the garment for work and came to be spoken of as the yoke of Christ. The hood or cowl belongs to the monastic choir. Dominicans wear a rosary at the belt, Carmelites brown, Cistercians white under a black scapular, Benedictines black.
Colour and cut therefore identify an order at a distance, which was the original point in a world where a traveller needed to know whose hospitality they were receiving.
Whether to wear it at all was a serious argument after the Second Vatican Council. Many active congregations of women adopted secular dress, on the ground that the habit distanced them from those they served, and others kept it for the opposite reason. Both positions are still held.