Underwriters Laboratory Incorporated is an organization located in the United States and is purely concerned with product safety performance. UL operates laboratories for the investigation of materials, products, equipment, etc. and sets standards which are aimed at reducing hazards affecting life and property.
In the case of printed circuit boards, the UL standards are: * U.L. 94 - Flammability rating * U.L. 796 - Performance Standards for Printed Circuit Boards
The UL 94 standard covers the operating temperature and flammability rating of the board. Most of OEM require UL approved boards and their fabricators apply a registered UL mark to every circuit to show that the board performs to this standard.