A flow is a sequence of steps that someone or something triggers: a click, a schedule, an incoming email, a call from another system, a value crossing a threshold. It has a start, an end and a visible path in between.
A rule has no trigger in that sense. It simply applies. A required field is always required. A calculation always follows from its inputs. A visibility rule always applies.
Many tools only know flows. There, even the obvious has to be rebuilt as automation, and the configuration becomes hard to follow over time: many small automations that are really rules, and nobody knows any more which one fires when.
In nexxcite, you define rules on the data model. They apply everywhere: in the interface, through the API and in flows running in the background.