The setting Manage purchase/manufacturing on project for parts?, under Part under the Inventory tab in the Settings procedure, must be checked to be able to use this function.
Here you can determine - at part level - from where part's material cost shall be loaded in the project accounting. For purchased parts the setting is called Purchase on project. If this setting is selected, the material cost such a part will then be loaded from the actual purchase and not from the reported material cost of the manufacturing order. This means that an exception is made for those parts in the post-calculation. For stocked, fictitious and packaging parts this setting is also available but is then called Manufacture on Project.
The setting Purchase/manufacturing on project for new parts, next to the setting mentioned above, makes this setting default when registering new parts.