Scapulars originated in the Middle Ages as full-length monastic work aprons. They were meant to help keep the religious habit clean when the monks were at manual labor. When the religious orders began to extend their distinctive spiritualities to lay people, they would frequently offern a symbol of affiliation.
The Brown Scapular, for example, was a sign of affilation with the Carmelite Order. Over time, the size of the “lay scapulars” began to to be reduced. As a Discalced Carmelite Secular (OCDS), I wear a large, ornate ceremonial brown scapular (ca. 8"x8" panels) to my OCDS Community functions, but a small ordinary one for everyday use. I was ritually invested in the large scapular when I was officially received into Formation in the Order. Our nuns and friars, of course, still wear a full-size scapular as an integral part of their religious habit.
Simply put, a small sacramental scapular represents in miniature the religious habit of a particular religious order. The Carmelite Scapular is especially a symbol of prayer, penance, and devotion to our Lady, and of the evangelical counsels of chastity, poverty, and obedience.