DXF internals documentation for CLASSES section
Classes Section
===============
+The CLASSES section contains CLASS definitions which are only important for Autodesk products, some DXF entities require
+a class definition or AutoCAD will not open the DXF file.
+
+The CLASSES sections was introduced with DXF AC1015 (AutoCAD Release R13).
+
+DXF Reference: `About the DXF CLASSES Section`_
+
+The CLASSES section in DXF files holds the information for application-defined classes whose instances appear in
+the BLOCKS, ENTITIES, and OBJECTS sections of the database. It is assumed that a class definition is permanently
+fixed in the class hierarchy. All fields are required.
CLASS Entities
--------------
-CLASS entities have no handle!
+DXF Reference: `Group Codes for the CLASS entity`_
+
+CLASS entities have no handle and therefor ezdxf does not store the CLASS entity in the drawing entities database!
+
+.. code-block:: none
+
+ 0
+ SECTION
+ 2 <<< begin CLASSES section
+ CLASSES
+ 0 <<< first CLASS entity
+ CLASS
+ 1 <<< class DXF entity name; always unique
+ ACDBDICTIONARYWDFLT
+ 2 <<< C++ class name; always unique
+ AcDbDictionaryWithDefault
+ 3 <<< application name
+ ObjectDBX Classes
+ 90 <<< proxy capabilities flags
+ 0
+ 91 <<< instance counter for custom class, since DXF version AC1018 (R2004)
+ 0 <<< no problem if the counter is wrong, AutoCAD doesn't care about
+ 280 <<< was-a-proxy flag. Set to 1 if class was not loaded when this DXF file was created, and 0 otherwise
+ 0
+ 281 <<< is-an-entity flag. Set to 1 if class reside in the BLOCKS or ENTITIES section. If 0, instances may appear only in the OBJECTS section
+ 0
+ 0 <<< second CLASS entity
+ CLASS
+ ...
+ ...
+ 0 <<< end of CLASSES section
+ ENDSEC
+
+
+.. _About the DXF CLASSES Section: http://help.autodesk.com/view/OARX/2018/ENU/?guid=GUID-6160F1F1-2805-4C69-8077-CA1AEB6B1005
-ezdxf needs for all entities handles and therefor store a :code:`(5, handle)` tag in CLASS entities, but do not write
-this handle tags into DXF files.
+.. _Group Codes for the CLASS entity: http://help.autodesk.com/view/OARX/2018/ENU/?guid=GUID-DBD5351C-E408-4CED-9336-3BD489179EF5
\ No newline at end of file
