For the parameter referenceTypeId, the spec says in part:
"...If not specified then all References are included and the parameter includeSubtypes is ignored."

There are two issues and 1 question:
1) "If not specified" is not defined.
2) "includeSubtypes is ignored"; does this mean that subtypes are included or excluded? which subtypes is this referring to?
Q: "includeSubtypes is ignored" only makes sense if "ignored" means that subtypes are included. Is this correct?
note: "all References are included" can be taken to mean both hierarchal and non-hierarchal but does not necessarily mean it includes subtypes.

We ran into a use case where the referenceTypeId on the wire was null (0), and the Server returned error "BadNoMatch" even though there was a Node at the specified BrowsePath. A NULL NodeId cannot be used in a server's address space, so using that as an argument to mean unspecified is counter intuitive.

related: what happens if a NodeId is used that is not null or a referenceTypeId? Does this count as unspecified, or if this is an error, Is there an error code for this?

The spec also says: "A text format for the RelativePath can be found in Clause A.2. This format is used in examples that explain the Services that make use of the RelativePath structure.".
The BNF does not include a symbol to denote "unspecified", and there is no example that shows an unspecified RelativePath

The BNF includes a mechanism for using a BrowseName for the reference type, so a client could specify "References", which is the root of all ReferenceTypes, both hierarchical and non-hierarchical, so why use "unspecified" at all? Is there a behavior difference between using "References" versus "unspecified"

There are many kinds of subtypes - data, objects, variables, etc., and not all are located below the "Objects" folder.
When using "unspecified" the parameter includeSubtypes is ignored. If this means they are excluded, then Is this referring to all subtypes, subtypes of ObjectTypes and VariableTypes only, or something else? If it applies to all subtypes, then TranslateBP2ID will not work with any of the subtypes below the Types folder.

Regards