Halliday & Matthiessen (2014: 252-3, 253n):
A fact is on a higher level of abstraction than an ordinary thing or an act. Ordinary things and acts are both material phenomena; they can be seen, heard and perceived in other ways. Thus while an act is more complex than an ordinary thing, it still exists in the same material realm. In contrast, a fact is not a material phenomenon but rather a semiotic one: it is a proposition (or sometimes a proposal) construed as existing in its own right in the semiotic realm, without being brought into existence by somebody saying it.
We could say that a fact is an act that has been propositionalised — that has been given existence as a semiotic phenomenon.