Every village has rules. Most of them are about manners. The three rules of Kagatsu are about survival, and they are stated in the same friendly tone as a request to return a borrowed tool. That tone is the horror - the rules are never framed as a threat, so you agree to them before you understand them.
The village is small enough that you can recognise almost everyone in it. The rule applies to the people you cannot place. Speaking to one is not an instant failure - it is more unsettling than that. The game treats the violation as an event, the conversation continues in a way that feels slightly wrong, and afterwards the village is not quite the same. Villagers who were friendly become briefly distant, and the village tightens around you a little more.
This rule is the most active one you can break, and the one that most directly shapes a run. It also creates the central tension of the farming loop: gifting produce is how you build relationships, but you need to have established someone as a neighbour before you can treat them as known. Trust is a resource you farm.
The boundary of Kagatsu is not marked with a wall or a fence. It is marked by the fact that paths simply thin out. The outskirts of the map are walkable, and the game lets you set out along them, but the village responds by making the return increasingly inconvenient - longer paths, closed gates, villagers who redirect you. Leaving is not blocked. Leaving is discouraged until you stop trying.
This is the rule with the most replay and progression weight attached to it, because it is the one that gates the larger map. The stretches beyond the village unlock as the story advances, and the story advances as you behave.
The simplest rule to follow and the easiest to break by accident. Evening in Kagatsu is where the game's pacing tightens: the day cycle closes down, the lighting shifts, and the soundtrack thins out to almost nothing. There are reasons to be out after dark - a late errand, a festival that runs long, something you heard - and each one produces a scene where the village is doing something it does not do during the day.
Because the rule is easy to break by accident, the game rarely needs to punish you hard. It simply shows you what the village is at night, and lets you decide what to do with that.
The three rules compound. Break the first and you have fewer people willing to tell you the second exists. Break the third and you learn things that make the first harder to interpret. The game tracks the state of each rule separately, and the ending you arrive at depends more on the combined shape of your violations than on any single dramatic choice.