ASTM E662 (the same test as NFPA 258) is ASTM's Standard Test Method for Specific Optical Density of Smoke Generated by Solid Materials.
The key thing to remember about this test, is that it measures smoke density in lab conditions, in a smoke chamber – but can not predict the density of smoke generated by the materials upon exposure to heat and flame under actual fire conditions.
The test measures the amount of light obscured by the smoke in a vertical chamber with a light source at one end. The testing material is ignited and tested in both flaming combustion and non-flaming test modes (smoldering while still putting off smoke). A photosensor at the other end measures how much the light is obscured by the smoke.
This test method can be used by manufacturers in research and development of materials but is not currently used for regulation or fire rating purposes.