This tank is from Movemama Apparel, let's see what they have to say:
Our Stripe Invisible Zip Breastfeeding Tank Top is designed for comfort and style. Made from 90% cotton and 10% spandex it features an easy-access invisible zipper for discreet and easy nursing and pumping. A versatile choice for everyday, this tank top is perfection worn alone or is amazing layered with other garments – making it amazing for any season or occasion. You will want all three colors in your wardrobe this season - we're sure of it!