Adding custom codes to an out-of-the-box typelist is a fundamental task in Data Model Configuration. To ensure that the configuration is upgrade-safe and follows Guidewire ' s architectural standards, developers must use the Typelist Extension mechanism.
The first rule is that base .tti (Typelist Internal) files must never be edited (ruling out Option D).
Modifications to these files will be lost during a platform upgrade. Instead, developers must use a .ttx (Typelist Extension) file. The filename must match the base typelist exactly (e.g., EmployeeAddress.ttx).
Adding _Ext to the filename itself (Option A) is incorrect and will prevent the application from merging the metadata correctly.
The second rule concerns the naming of the new typecode. According to the InsuranceSuite Developer standards, any code added by a customer to a Guidewire-owned typelist should include the _Ext suffix (e.g., alternate_Ext). This " namespacing " protects the customer from future collisions. If a future release of PolicyCenter or ClaimCenter includes a base alternate code in the EmployeeAddress typelist, the customer ' s version will remain distinct, preventing database errors or logic failures during the upgrade process.
By creating the .ttx file and using the _Ext suffix on the typecode (Option C), the developer adheres to the Open Type System principles. This ensures the custom data is properly recognized by the UI and Gosu rules while remaining perfectly aligned with the Guidewire Cloud Delivery Standards.