More accurate description to Blosc2 format

README_B2ND_FORMAT.rst

@@ -1,7 +1,10 @@
 b2nd Format
 ===========
 
-The b2nd format is just a `b2nd metalayer <https://github.com/Blosc/c-blosc2/blob/main/README_B2ND_METALAYER.rst>`_
+The b2nd format is meant for storing multidimensional datasets defined by a shape and a data type.
+Both the shape and the data type follow the NumPy conventions.
+
+It is just a `b2nd metalayer <https://github.com/Blosc/c-blosc2/blob/main/README_B2ND_METALAYER.rst>`_
 on top of a Blosc2 `cframe <https://github.com/Blosc/c-blosc2/blob/main/README_CFRAME_FORMAT.rst>`_
 (for contiguous storage) or `sframe <https://github.com/Blosc/c-blosc2/blob/main/README_SFRAME_FORMAT.rst>`_
-(for sparse storage) for storing multidimensional information.
+(for sparse storage).