The scapular has two quite different lives, one as part of a monastic habit and one as a small devotional badge worn by ordinary Catholics, and the second grew out of the first.
The garment of work
In its original form the scapular is a long panel of cloth hanging front and back over the habit, and the Rule of Benedict names it as the garment for work, worn so that the fuller habit is protected. It became a fixed part of the dress of many orders, received at profession, and in some it carries the symbolism of the yoke of Christ taken up by the monk or nun. Worn thus it marks a member of a religious order at labour and at prayer.
The small scapular
From the monastic garment developed the small scapular, two little squares of cloth joined by cords and worn under ordinary clothing, by which a layperson is enrolled in the devotion associated with an order. The best known is the brown scapular of the Carmelites, tied to a promise of protection reported in a medieval vision, and enrolment is a formal act. The small scapular thus lets a lay Catholic share symbolically in the life of an order without entering it, which is why a monastic work-garment ended up as one of the commonest sacramental objects worn on the body.