The ASTM E84 (UL 723) cover the Surface Burning Characteristics for Flame and Smoke development
Pipe hangers and pipe supports are also covered by standards that apply to building materials such as ASTM E84 standard test method for surface burning characteristics of building materials. To my knowledge E84 is a test that is designed for sheet testing of surface finish materials and does not provide for the testing of shapes such as pipe and fittings or fixture assemblies.
UL 2043 covers discrete products installed in air-handling spaces (plenums) as defined in Article 300 of ANSI/NFPA 70, "National Electrical Code," Chapter 4 of ANSI/NFPA 90A, "Standard for the Installation of Air-Conditioning and Ventilating Systems," Section 602 of the "International Mechanical Code," and Section 602 of the "Uniform Mechanical Code."
Question is to what standard can one approve discrete products such as plastic pipe, pipe hangers and supports in a plenum; both standard tests or the more dedicated UL 2043?