An InstanceDeclaration is an instance (cf. https://reference.opcfoundation.org/Core/Part3/v105/docs/3.1.6)

Since it is an instance, all mandatory nodes must be "attached" to it (cf.: https://reference.opcfoundation.org/Core/Part3/v105/docs/6.4.2)

This description in Part 3 6.4.2 should be improved:
The Nodes within the newly created hierarchy may be copies of the InstanceDeclarations, the InstanceDeclaration itself or another Node in the AddressSpace that has the same TypeDefinitionNode and BrowseName.

Could you explain or enhance the statement please?

To me, it is fine (and encouraged) to do this:
Have a first VariableType
Add a node "A" (mandatory) to the VariableType

Create a second VariableType
Add a Node "B" to it, using the first VariableType
"B" has a copy (reference to) of node "A" to fully declare the InstanceDeclaration, because "A" is mandatory.

There has been some arguing around whether this is correct or not.