Fitting one thing carefully into another to denote a change–a splash of colour, a change of texture, of direction or a section of fullness or gathering such as a godet or gore. The effect might be subtle and depend on light and texture for definition, or be a very obvious insert.
Trimmings of an alternative colour or texture [such] as narrow strips of fabric, ribbons, even medal bands can be inserted into the main fabric to personalize and add stylistic detail to any type of furnishings. A piece of lace inserted between the seams of underwear was considered very glamorous in the 16thC, when lace first became widely used.
As a method of joining separate materials, inserting a new fabric between two others shows and defines their separate identities. For example a thin strip of bright yellow might unite fabrics of red and blue on either side; or a pink and white floral between two pieces of pink and white.
Braid and or contrast materials can be inserted between a short curtain and a hem border to make it longer, for practical reasons that can be disguised as a design choice.
See godet, gore..