The cost comes from the transactions you apply these forms of “behaviour” against.
If you were to manufacture 100 and you set a rate of say $1 the fixed would apply $1 in total, whilst variable would apply $100. It really depends how you configure it and where, which of course depends upon what the customer needs. Also as Technet will tell you “The assignment of a behavior only serves a reporting purpose. For example, costs can be displayed with segmentation of fixed and variable costs on the costing sheet, and in the Cost rollup by cost group form.”. If no behaviour is assigned then variable is assumed.