There is no way to identify when or how much a Purchase Code is used across multiple projects or how many and to what extent different Suppliers are involved.
Adding new report where the details of ' Expenditure by Purchase Code' include a Project field and where the Invoice Number and Supplier are separate fields.
Sorting the data by each field and providing sub totals when sorted by project or supplier would make the data held within the system much more accessable and useful. CSV export is always helpful.
The same could also be applied to 'Income by Sales Code'