Categories serve as short descriptions (on the invoice) for specific expenses. You can create new ones when editing or creating an expense.
Please watch this video or follow the steps below:
1) Create a new expense, or edit an existing one.
2) Click on the "Category" dropdown menu, and select "Manage Categories".
3) Click on "Add New Category".
4) Give the category a name, and hit "Save".
What happens now?
Your newly created category will now be selectable for all future expenses!