Idea of Not Applicable Icon
It's a logo for the words "Not Applicable". It is a box with rounded corners. Inside the box are the letters N and A. Both of them are capitalized.
This Not Applicable icon is made in Material Design style
developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 6,200 icons, all in the same style and quality.