"Show on Invoice" in WHMCS refers to the option to display a particular custom field or specific information on the generated invoice that is sent to the client. This allows you to include additional details or special instructions that may be relevant to a specific order or client.

For example, if you have a custom field like "Special Instructions" or "Additional Information," you can choose to display it on the invoice so that clients can see any specific notes related to their purchase.

Here are the steps to set a custom field or information to be shown on the invoice in WHMCS:

1. Log into WHMCS:

   Access your WHMCS admin panel using your administrator credentials.

2. Navigate to Custom Fields:

   • Go to "Setup" in the top menu.
   • Click on "Custom Client Fields" for client-related fields or "Custom Fields" for product-related fields, depending on the type of field you want to configure.

3. Edit the Custom Field:

   • Locate the custom field or information you want to show on the invoice and click on its name.

4. Configure the Field:

   • In the field details, you should find an option like "Show on Invoice" or similar.
   • Enable or select this option to indicate that you want to display this field on the invoice.

5. Save Changes:

   • Click the "Save Changes" button to apply the setting.

By setting a custom field or information to be shown on the invoice, you ensure that clients can easily access important details related to their order when reviewing their invoice.

This functionality can be particularly useful for providing specific instructions, contact information, or any other relevant information that clients should have easy access to when reviewing their invoices.