The application layer is also known as a logic tier, as this is the core of the product where all the business logic resides. The presentation tier collects the information from the user and passes it to the logic tier to process and get a result. For example, on an e-commerce website such as Amazon.com, users can enter a date range on the order page of the website to find their order summary. In return, the web layer passes the data range information to the application layer. The application layer processes the user input to perform business logic such as the count of orders, the sum of amounts, and the number of items purchased. This returns information to the web layer to render it for the user.

Generally, in three-tier architecture, all algorithms and complex logic live in the logic tier, which includes creating a recommendation engine or showing personalized pages to the user as per their browsing history. Developers may choose to implement this layer in a server-side programming language, for example, C++, Java, .NET, or Node.js. The application layer performs logic on the data, which is stored in the database layer.