The factual (or fact) dimension is about drawing a distinction that allows an observer to indicate one thing while ignoring something else. Sometimes Luhmann used the term fact dimension and other times he used factual dimension.
One can speak of the fact dimension in relation to all objects of meaningful intentions (in psychic systems) or themes of meaningful communication (in social systems). . . . The fact dimension is thereby constituted in that meaning divides the reference structure of what is meant into “this” and “something else.” Thus the point of departure for a factual articulation of meaning is a primary disjunction, which contrasts something as yet indeterminate with something else as yet indeterminate. (Luhmann, Social Systems, 76)
The “this and something else” is a two-sided form. The famous gestalt image of the vase face demonstrates the two-sided form.
An observer can see a vase or two faces sequentially but not both simultaneously. In other words, there is a conflict. One can use the temporal dimension to see first one image and then the other, switching back and forth. Crossing the two-sided form of “this and something else” takes time. Crossing does not mean crossing a boundary; it means approaching one horizon until one must turn back. A horizon cannot be reached; so it of course cannot be crossed.
The social dimension introduces the difference between consensus and dissension, or consent and dissent. People often speak of getting to or reaching consensus. But dissension is undesirable in itself. The social dimension features the possibility of crossing from consensus to dissent and back again. Again, these are horizons not crossable boundaries. Communication can move toward dissent, but at some point it must turn back. This is what autopoietic closure means.
As Luhmann puts it,
A horizon is not a boundary; one cannot step across it. At some point one must turn back, and the opposite horizon indicates the direction “back.” (Social Systems, 77)
The possibility of dissent guarantees structural instability.