Solution

You will need to configure the Vendor settings as follows;

1. Go to Administration >> Vendors and click Get Data
2. Click on to open the Vendor Details window
3. Click on the Other Settings tab
4. Check Show next appointment details in the invoices?

Note: By default, this option is turned on but you can manually disable it.

5. Once this check box is ticked, each time a Receipt or Invoice is generated, you will be presented with the client's next appointment details as shown below:

For more information see: How to Generate a Receipt for a Cash Sale, How to Create a Multiple Payment Receipt, How to  Create a Client Invoice, How to Create a Debtor Invoice