Sometimes, you have to spend money on your customers. Make sure you’re billing them for it.
Usually, money flows from your customers to your business. But there may be times when you have to
purchase items for a job whose costs will eventually be reimbursed. Or you, or an employee, might spend time providing services for customers and get paid for those hours by your company before you receive payment from the responsible party. If you’re a sole proprietor with no payroll and no reserves, of course, you just have to wait to be paid for your work.

In the first two cases, you’re spending money upfront that will eventually be paid back. In all three cases,
QuickBooks Online calls these billable expenses and billable time, and it does a good job of tracking these transactions – much better than if you were scribbling notes on a receipt or a paper timecard.