What's the difference between a Purchase Invoice and Expense?
If you've received an invoice from a supplier then we'd suggest adding a new Purchase Invoice. If you've bought something smaller using your own funds, or using a company card and then record it as an expense.
Accounting-wise Purchase Invoices and Expenses are exactly the same.
Adding expenses on to the system is designed to be quick and easy. Payment details are recorded at the same time. You don't want to have to raise a purchase invoice each time you buy a small item, like a taxi fair, so you can add an Expense for these items.
Purchases are assigned to a contact and you can come back and record the payment details at a later date.