I concur with renosteinke. For example, IBC requires a suspended ceilings in siesmic areas be designed to support the ceiling, lights, ducting, sprinklers, and anything attached. Does this mean that the ceiling can support the lights? Not yet. There is still the project specs, state, and local ordinances, before it gets to the NEC. One you get to the NEC you still need to ensure the clips or rivits are listed. If you are going to screw, bolt or rivit to the grid, you want to verify that grid is designed for that. When using the fasteners, you know darn well the the t-grid will get flatten from the riviter or tek-screws from the cordless. That will effect the structual integrity of the t-grid. Each project I was on, the fixtures were supported seperately from the T-grid. Then again I live in an active siesmic zone. Local fire code may require supporting the fixtures too.
If the ceiling is existing, the same rules applies, except the ceiling may not be able to support the lights. I did a retro project where the first light I popped out, the ceiling darn near come down. the ceiling was supported by speaker wire. The wire in the are snapped and tention on the fixture was keeping everything together.
