You can create custom part subtypes to fit the needs of your site. These new part subtypes must use appropriate structure-level properties, and the relationship constraints you define must be allowed by the Windchill Aerospace and Defense configuration management methodology. For example, you cannot create a part subtype with a structure-level property of Lower Level Part and define a relationship constraint allowing this part subtype to have a child subtype with a structure-level property of Configuration Item. For more information, see Defining Part Subtypes and Relationship Constraints.