Expenses can be tracked using the three lists available for project time tracking (Customer, Project, and Account Code), but there are two additional lists available only for expense tracking: Vendors and Events.
Vendors are generally entered as the recipients of payments (e.g., hotels, distributors, restaurants, gas stations, etc.). The Events list serves as an additional layer of classification for expense entries that can be useful for grouping expenses. For example, all expenses related to a trade show could be grouped by creating and using an event specific to the show.
Please refer to this article and this article for detailed instructions regarding the setup and use of these lists.