Subtypes are a subset of features in a feature class, or objects in a table, that share the same subtype attribute field value. They are used to categorize your data. For example, you can categorize the streets in a city streets feature class into three subtypes: local streets, collector streets, and arterial streets.
Feature layer subtypes in ArcGIS for AutoCAD are treated the same way as any other feature layer. Each subtype includes its own AutoCAD layer and set of properties. Subtypes may have their own sets of coded value domains and range domains on fields that pertain to that subtype only. You edit a subtype geometry the same way you edit a parent feature layer.
The membership of a feature in a feature layer with a subtype is determined by a subtype field designated by the author of the geodatabase. Subtype fields establish the membership of a feature within a subtype of the feature layer based on the value of that field.
Subtype fields are established by the GIS administrator on geodatabases that are shared through web services as web feature layers in the application. You can include those field definitions on document feature layers extracted from web feature layers or from feature classes exported from ArcGIS desktop to AutoCAD files as document feature layers. You cannot establish new subtype definitions on a document feature within the application.
The modification of a feature's AutoCAD layer automatically updates the features subtype field value. Conversely modifying the subtype field's value changes the AutoCAD layer of the feature entity effectively managing its subtype membership. Subtype fields are governed by a field with a coded value domain.