Some datasets can have a production volume of 0. That means that the reported production volume for the given region is indeed 0, and the activity does not take place. Such datasets may eventually simply be removed from the database, since they describe activities that no longer happen. However, sometimes they remain in the database, for example for legacy purposes (some user projects may depend on them as inputs). Other examples might be datasets for future technologies.
Without a production volume, these datasets will not be included in any market datasets, and they will not be used within other parts of the ecoinvent database unless explicitly selected by the dataset creator, for reasons specified in the documentation.