Reusable Objects
Most features in FluidCAD consume their inputs. When you extrude() a sketch, the sketch geometry is removed from the scene so it can't accidentally be picked up by the next feature. The same applies to selections used by shell(), cut(), sweep(), and so on.
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.