Add section about how the modelling should work

content/quickfacts-1/index.md

@@ -61,6 +61,18 @@ TODO What to do if at the "head" of a beltline
 ### Inserter-Belt
 TODO
 
+# Modelling
+Each node essentially has a set of input and output "slots", which have different priorities.
+
+Each slot can have filters attached, from both the upstream and downstream side.
+Filters from the upstream side are mostly dependent on configuration, like filters on splitters.
+Filters from the downstream dependent on their slot filters and also their type, like machines will apply filters to their upstream slots that constrain it to items that it needs/can process.
+For transport nodes (belts, inserter, etc.) they should forward their downward filters and combine them with their own filters
+
+To collect input items, a node goes through its input slots in decreasing priority and collects everything that fits its own criteria.
+
+To distribute the items, a node goes through its output slots in decreasing priority and puts as many items as possible in that slot.
+
 # Running it manually
 
 {% carousel() %}