striped stretch mesh, it may be suited to bra wings but for larger sizes ensure your elastic is strong enough.  This powernet is quite strong, but not as stretchy as some (bra wings do not need to be super stretchy provided you use a bra fastener).

It is ex-manufacturer, used on both bra wings and cups (over a foam cup)

The 'White' is undyed, so not a pure white.

Most probably polyamide elastane