Reusable Objects
Most features in FluidCAD consume their inputs. A selection used by shell(), fillet() or project() is gone afterwards, so it can't accidentally be picked up by the next feature.
Units
Numbers in a file are in that file's unit. The unit is metadata: it is not a scale factor applied to your numbers, and nothing converts a value on its way from your source to the geometry. extrude(30) in a millimetre file is 30 mm of extrusion; in an inch file it is 30 in.
Project Parameters
A project parameter is a param() declared once, at the top level of a *.params.js file, exported, and read from any part or assembly through an ordinary import. It is the home for a dimension that several files share — a wall thickness, a bolt size, a clearance — so the number is written in one place and every model that reads it rebuilds when it changes.