To build an assembly, drag and drop items into it and associate each item with a quantity. Quantities can be fixed numbers or variables. See Formulas for information about variables.
This is either a fixed quantity or a variable. To enter a fixed quantity select "<Fixed Quantity>" from the dropdown list and then type a number in the text field that appears. To enter a variable select it from the dropdown list. The variables that appear in the list are determined by the formula selected in the header.