For types, which represent a collection of bits, OPC UA offers three possibilities:

1) The OptionSet DataType defined in Part 3
2) The OptionSetType VariableType defined in Part 5
3) Just a DataType derived from some UInteger DataType with an OptionSetValues property

While the 3rd possibility is often used in namespace 0, it is barely visible in the specification. Only in Part 3, Table 16, at the OptionSetValues property, does it say "applies for OptionSet DataTypes and UInteger DataTypes". It's just these three words. And in Part 5, OptionSetType mentions the OptionSet DataType and gives advice when to use what, but does not mention the third possibility.

Suggestion: Create a general chapter about "Types which represent a collection of bits", e.g. in Part 3, chapter 8.1. Mention the three possibilities and give advice when to use what. Then the paragraph with the advice in Part 5 at OptionSetType can be removed.