Any object in an HDF file can have annotations associated with it.
However, with the SD and Vset (VData/Vgroup) interfaces, user-defined
attributes are available, which can be used instead.
There are a number of types of annotations:
Labels are assumed to be short strings giving the "name" of a
data object.
Descriptions are longer text segments that are useful for giving more
indepth information about a data object.
File annotations are assumed to apply to all of the objects in a
single file.