Given a reference,
ref_ptr, to an attribute attached to an object, H5R_OPEN_ATTR opens the attribute attached to that object and returns an identifier.
rapl id is a reference access property list identifier for the reference. The access property list can be used to access external files that the reference points to (through a file access property list).
aapl_id is an attribute access property list identifier for the referenced attribute.
The attribute opened with this function should be closed with H5A_CLOSE when it is no longer needed.