Both an expense and a bill are used to record an expense amount, albeit in different scenarios.
Let’s say you incur an expense and pay for it then and there. Record it as an expense in Zoho Books. An example of this would be money spent on a luncheon with a prospective client.
On the other hand, if the expense is one that doesn’t require to be paid until later, you need to keep track the amount you owe till it’s paid off. You can do this by recording it as a bill. An example is the rent you pay for your office space.