The class identifier. To a list of the HDF5 class types please refer to the Datatype Interface API help.
But the what I could find on the Datatype Interface API at https://support.hdfgroup.org/HDF5/doc1.6/UG/11_Datatypes.html does not give a clear answer. For example, I created an attribute using H5LTset_attribute_string. I would have expected the type_class for that to be H5T_STRING but H5T_STRING (in my fortran program) has a value of 216172782113784384 but type_class was set to 3 by H5LTget_attribute_info.
I figured out the problem. It turns out that H5T_STRING will not work under Fortran. What needs to be used is H5T_STRING_F. And if I follow the link you give to the https://portal.hdfgroup.org/display/HDF5/H5T_GET_CLASS page I do not see any mention of the fortran equivalents of the class identifiers.
The problem is that while this is documented somewhere it is very easy to miss. Thus, for example, a link from the https://portal.hdfgroup.org/display/HDF5/H5T_GET_CLASS page the page where the fortran version is discussed would alleviate some frustration for someone who is new to HDF5 programming.