1. Add custom field
Custom fields can be added via Settings Custom fields and there via the relevant tab.
For the use case example, we go to the Article tab and add a number field, which we call "EAN". To display the custom field on its own tab, enter a new tab "EAN".
INFO: Each custom field gets a unique placeholder from CashCtrl in the form of $customFieldXY. This placeholder can be used to display the value of the custom field e.g. on documents.