Each Appian design object serves a distinct purpose in application development. A Record Type is a cornerstone of Appian's Data Fabric, designed to unify and present a holistic view of business data from various enterprise sources. A Process Model graphically defines a business workflow, orchestrating the sequence of human and system tasks. An Interface is used to build the user-facing forms and dashboards that display information and capture user input. Finally, an Expression Rule encapsulates a piece of reusable business logic or calculation, which can be called from other objects to ensure consistency and maintainability.

Create a new data type: This describes the function of a Custom Data Type (CDT), which is a separate design object used to define a specific data structure.

1. Record Type: Appian Documentation, "Record Type Object". This page states, "A record type is a design object that you use to define a holistic view of your business data... Appian Records allows you to unify data from across your enterprise."

2. Process Model: Appian Documentation, "Process Model Object". This source explains, "A process model is the primary tool for defining a business workflow in Appian. It is a graphical representation of your business process..."

3. Interface: Appian Documentation, "Interface Object". The documentation specifies, "Interfaces are the primary way you can show users your applications and data... You'll use interfaces to display information to users and to allow them to enter information in your applications."

4. Expression Rule: Appian Documentation, "Expression Rule Object". This document notes, "Expression rules are used to store a single expression that can then be called from other expressions... This reusability saves development time and improves application maintainability."

5. Custom Data Type (CDT): Appian Documentation, "Custom Data Types (CDTs)". This reference clarifies, "A custom data type (CDT) is a developer-defined data structure. CDTs are used to store a collection of related data in a single variable." This confirms that creating a new data type is the function of a CDT, not the other listed objects.