Tables & field types — Connect tables with Relation, Lookup, and Rollup (3)

A condition can also test something one link past the records it is filtering — the dropdown lists those as Order → Type. That is what lets a discriminator live where it belongs: model purchases and sales as one Orders table with a Type, and a product can still total Sum of Quantity across Order Lines, only where Order → Type is Purchase. Without it you would have to copy the type onto every line just to make the filter legal. Pair two such rollups with a formula that subtracts them and a reorder level, and stock keeps itself up to date. The config appears automatically when you pick one of these types in the Type dropdown. If the table you want isn't linked yet, choose + New link to… at the bottom of the route picker — the Relation field is created for you as part of saving the Lookup or Rollup.