In CEH v13 Module 12: Hacking Web Applications, the N-tier architecture is explained as a model used in web applications, typically with:

Presentation tier – User interface (UI) that interacts with the user.

Logic tier (also known as application or business logic layer) – Processes commands, makes logical decisions, and handles data movement and processing between the UI and the database.

Data tier – Manages database access and data storage.

So, the Logic tier is responsible for:

Handling the business rules.

Processing user inputs.

Accessing and manipulating data from the database.