From Sewing Chest:
This is the non-stretch fabric used on the inside of bras to stabilise the outer fabric, in this way you can use stretch fabrics but still make a firm supporting bra. Double up for extra support on the lower cup if needed. This has a greater opacity than some of the other net fabrics and has a smoother/softer feel, there is still the small amount of 'give' to ensure a smooth line.
56gsm