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.
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!
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