Restrained and Unrestrained are structural conditions. “Restrained” pertains to a structural system that takes into account thermal expansion, whereas, “Unrestrained” (more common) does not. UL recommends that you confer with your structural engineer to confirm the system appropriate for your project.

The UL Code Authority newsletter article Restrained and Unrestrained Assemblies – What Designers and AHJs Need to Know contains more information on this subject.