A fact is one remembered thing with a source and permissions attached: 'Acme was promised SSO by Q3, from the security review doc.' Facts have scores, weak facts stay out of answers, and wrong facts get retired with the reason on record.
The unit of memory matters. Because every fact is discrete and sourced, Heidi can show receipts for any answer, retire exactly the fact that turned out wrong, and keep a 1:1 conversation out of answers to people who were not in it.
Facts attach to entities, the people, companies, projects, and systems Heidi knows, which is what makes cross-source answers possible.