For OPC UA Clients it has been decided that the implemented functionalities will differ that much, that it doesn't make sense to define multiple OPC UA Client profiles. Therefore the only UA Client profile that is currently defined is the "Standard UA Client 2017 Profile". Because this profile for example lists the "Attribute Client Write Base" facet as being mandatory, not all clients can fulfill the requirements of the profile. Due to this fact, we are currently certifying client rather for facets than clients against this specific profile. Now we have been asked what the absolut minimum of facets is, a client needs to comply with in order to get certified. Right now there is no minimum defined somewhere and a client could for example just ask to get certified for the "UA-TCP UA-SC UA-Binary" facet. In this case of course we would verify any other implemented feature for being compliant but what happens for desired behavior which is defined in the spec but the test case for this behavior is defined in a facet the client doesn't like to get certified for? One example for this is the desired behavior of an UA Client in case the UA Server changes its state to SHUTDOWN_4. According to the specification the client needs to disconnect and clean up resources. This is being tested in the "Base Client Behaviour" facet. So would a client fail the certification testing for not being compliant to the specification or would it succeed because the test case is not required for the chosen facets?

In order to solve this problem I do recommend to define a minimum or base UA Client profile which identifies the absolut minimum a client needs to implement in order to be compliant with the specification. A client then still can choose additional facets according to its needs and implemented features. With such a base client profile being defined, we can change the definition for the certification program in order to mandate at least one profile for any UA interface a product needs to comply with.