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One thing that has always been clear about the OSI model is that the Application layer includes application data and application protocols. The Hypertext Transfer Protocol (HTTP) is an application protocol that a web browser uses for communicating with web servers. Application data would be an HTTP GET request that the browser sends to a web server. Likewise, the web page that the server sends in response would also be application data. In short, application data is whatever the application sends or receives over the network.

Incidentally, an application can use more than one protocol. For example, when a web browser uses the Hypertext Transfer Protocol Secure (HTTPS) protocol to send a request to a web server, it's making use of two protocols: HTTP and Transport Layer Security (TLS). Despite the latter's confusing name, both are application protocols.

For all practical purposes, the upper layers (Session, Presentation, and Application) are one layer: the Application layer. The actual functions of the Session and Presentation layers—things like authentication and negotiating an application protocol—occur in the application anyway. They don't include any network functions and are concerned only with application data and application protocols.