The Custom properties field feature helps you add additional information about your client data, which may appear or not on your documents.
To set them up, go to 'Clients' in the menu.
Then click on the edit icon to the right of the client's name.
Select 'Custom Properties', then 'Add custom field'.
The 'Name' and 'Value' fields will then open, which you must fill in. You can also choose if you want or not this additional information to appear on your invoices and documents. When you are done, click 'Update the client'.
You can add as many fields as you want and delete them whenever you want by clicking on the 'X' in the image.