Users are able to select which of the existing custom fields in the board will become visible on the closed card view (img.1).
This is particularly useful for teams that have a great number of custom fields and use some of them more frequently.
As a next step, you have to checkmark Show custom fields and then you have to select Custom (1).
This opens an additional window with the list of all existing custom fields (2) that belong to the board. Select only the fields that you want to display and save your changes.
Note: If you want to display all custom fields, then you have to select: All.
Important: the fields will become visible upon refresh only if the custom fields are marked as mandatory. Otherwise, you have to manually add them to the card from the Custom fields menu at the top right side of the opened card.