Project activities can be used to execute/perform work on a sub contract. Therefore, project activities can be connected to line items of a sub contract. However, only one activity can be defined as the default activity for a sub contract. This default activity will be used to report cost from all the items that have not been connected to an activity. A Project activity can be connected to the sub contract in the Project tab. Alternatively, you can directly add an activity to the item, along with the project, which will be updated in the Project tab.