Hi,
The restrictions area uses the "AND" logic.
So, if you restrict a custom field to a specific user group with the "access level" setting, and if also select a category, then the custom field will be ignored unless the product is in that category AND the current user's user group matches with the user group selected.
Similarly, if you restrict a custom field to a category and to a product, then the custom field will be ignored unless the current product matches with the selected product AND one of its categories is also selected in the categories setting.
Note that you can create an unpublished category. Select it as a second or third category in your products, and then select that category in the custom field's categories setting. That way, the restriction will be enforced and the category won't be visible on the frontend. You can even have both the "stickers" category and that unpublished category together in the categories setting.