Class type is a way to link object types and classes → a class must always be linked to a single class type. With another words – Class type represents the object type for which the class is being created for.
E.g. Class type 001 (predefined by SAP) is linked to material, 002 is equipment, 023 are batches, … So when you create a class for class type 001, then characteristis included in that class will be used to describe a material.
There are many class types already predefined in SAP standard, but you are allowed to create your own types in IMG → Cross-Application Components → Classification System → Classes → Maintain Object Types and Material Types. You just have to select the desired object type (or add the object type if it’s not already listed) when creating new Class Type.