The khimar covers the woman’s head and hair, while the khirqah binds across the chest; they are not interchangeable.
خِرْقَةٍ تُرْبَطُ بِهَا ثَدْيَاهَا
Translation: A strip of cloth with which her breasts are secured.
The wording identifies the band’s specific place, distinct from the head-cover and outer sheet.