Building the list of things you are measuring, and how each one turns a measurement into a quantity.
An item is one line of the takeoff: a thing being measured, with a unit to report it in. This is how the list is built and priced.
Start with Adding items. The rest is the machinery behind the number an item reports: the unit it converts into, the formula that conversion is shown as, and assemblies for when measuring one thing should measure several.