Facts are the scaffolding. They are dates, places, names, recorded events, material culture, and primary sources that can be pointed to.
Facts are also negative space: what the person could not have known yet, what had not happened, what technologies did not exist, what maps were wrong, what rumours travelled faster than letters.
A good Facts layer includes the limits of perception, because history is often less about what happened and more about what was believeable at the time.
# See - Perspective and Facts - Hard Science Fiction - Historical Method and Primary Source