The best known of the small scapulars is the brown scapular of Our Lady of Mount Carmel, and its history is a case study in how a devotion, a legend and an official correction interact.
The tradition holds that the Virgin appeared to Simon Stock, an English Carmelite, at Cambridge in 1251, gave him the scapular, and promised that whoever died wearing it would not suffer eternal fire. The historicity of the vision is disputed by historians, including Carmelite ones: the earliest surviving account of it is considerably later than the date given, and the details of Simon Stock's own life are thin.
A second and larger claim attached itself later, known as the Sabbatine privilege: that the Virgin would release from purgatory on the first Saturday after death those who wore the scapular and met certain conditions. It was attributed to a bull of John XXII, and the authenticity of that document has been seriously questioned for centuries.
The church's response was neither to endorse nor to suppress but to restrain. In the early seventeenth century the Holy Office permitted the Carmelites to continue preaching the devotion while restricting what could be claimed for it, and modern Carmelite teaching presents the scapular as a sign of consecration to Mary and of a commitment to a way of life, explicitly not as a guarantee attached to a piece of cloth.
That framing matters, because the alternative reading is close to the definition of superstition, and the order has said so.
Five scapulars are commonly listed as approved, of which the brown is the most widespread. Enrolment is by a priest, once, after which the wearer may replace the cloth themselves.