After there have been several discussions on handling of optional features in conformance units I would like to get the issue clarified. The last discussion was in an overall Harmonization working group meeting .

Discussion topic: Is it allowed in a conformance unit to define optional things or is it everything defined in a conformance unit mandatory, and optionality can only be achieved on a higher level (Facets)?

Current status:
Part 7 defines Conformance Units as:
Terms section:
a specific set of OPC UA features that can be tested as a single entity
Note 1 to entry: A ConformanceUnit can cover a group of services, portions of services or information models.
Section 4.2 ConformanceUnit
Each ConformanceUnit represents a specific set of features (e.g. a group of services, portions of services or information models) that can be tested as a single entity. ConformanceUnits are the building blocks of a Profile. Each ConformanceUnit can also be used as a test category. For each ConformanceUnit, there would be a number of TestCases that test the functionality described by the ConformanceUnit. The description of a ConformanceUnit is intended to provide enough information to illustrate the required functionality, but in many cases to obtain a complete understanding of the ConformanceUnit the reader may be required to also examine the appropriate part of OPC UA. Additional Information regarding testing of a ConformanceUnit are provided in the Compliance Part 8 UA Server or Compliance Part 9 UA Client test standards. The same features do not appear in more than one ConformanceUnit.
REMARK: I could not find Compliance Part 8 and 9 to download, the link only leads to Part 1 to 5.

We do have released conformance units in the base spec like:
Data Access AnalogItemType -> Support AnalogItemType Variables with corresponding Properties. The support of optional properties will be listed.
Base Info Placeholder Modelling Rules -> The Server supports defining custom Object or Variables that include the use of OptionalPlaceholder or MandatoryPlaceholder modelling rules.
REMARK: Will be replaced by „Base Info Base Types“
Base Info OptionSet -> The Server supports the VariableType OptionSetType.
REMARK: OptionSetType has an optional Property called BitMask

These are just a few examples of the base specification. Many companion specifications are defining Conformance Units in a similar way.