A hard assigned construction is when you directly assign a construction to a surface or sub-surface. This is what is done if you work with an IDF file. You can also do this with an OSM file, but the more common approach is to create a construction set that is set at the building object, story, or space type. This make global changes very easy and still allows you to override a specific surface by assigning constructions directly as needed. This diagram on the OpenStudio website visually shows where you can assign constructions. The two two red dots in the construction column would be 'hard assigned' while the yellow dots are inherited.