In-App Extension involves customizing or extending SAP standard applications within the SAP core system itself (e.g., within SAP S/4HANA).

Where it runs:

• Inside the same SAP S/4HANA system
• Uses SAP tools like Key User Extensibility tools, ABAP in Eclipse, or Fiori App Customization

Typical Use Cases:

• Adding fields to an existing app
• Creating custom logic using BAdIs or user exits
• Modifying UIs using SAP Web IDE or SAP BAS
• Enhancing CDS views
• Adding validations or business logic via extension points

Pros:

• Tight integration with core processes
• No need for middleware
• Real-time access to SAP data
• Simpler for smaller extensions

Cons:

• Might impact system upgrades
• Limited by the extensibility options SAP provides
• Less flexibility for building completely new applications