This issue was raised by Nicola Guarino during the final CPSV-AP v2.0 revision webinar. He asked to include the service user to the specifications, and change the cardinality of the service provider property from 0..n to 1..n.
In addition, he added that there are constraints concerning roles that cannot be modelled using only the participation class. For instance, the delegation relationship between the competent authority and the provider.
We propose to have a look at the usage of service user and provider properties and discuss through the mailing list.