TCP/IP has four defined layers, the application layer, transport layer, internet layer, and the link layer. Each layer does something with the data, then passes that data on to the next layer for additional handling. None of the layers deal with the data from the other layers directly, they just add or handle their specific part of the data and then pass it on.
Ignoring the real-world complexities involved, consider the following analogy. Let's suppose you want to write a letter to your friend in another country. First, you write the letter (application layer), put the letter into an envelope (transport layer), address the envelope (internet layer), and then put in the mail box (link layer). From there, the letter travels to its final destination via mailperson, then by post office, then by other means (e.g. trucks or airplanes), which then goes to the far side post office, then to the mailperson, then to the mailbox, and finally to your friend.
The TCP/IP design means that, as long as the boundary rules are followed, you can swap out one implementation of any layer with another without disrupting the entire stack. As an obvious example, the link layer might be: WiFi, Ethernet, microwave, IR, fiber, dialup, etc. The internet layer doesn't care how the link layer is configured, just that it needs to route data from point A to point B. It also doesn't care what data is in the packet created by TCP/IP, it's just routing data across the network we call the Internet.
So, you wouldn't say that one layer is a subset of another, they're all distinct layers with specific purposes. Each layer is a part of a greater whole that makes internetwork data transport possible. The four layers are all required for an application (ignoring some special cases, like pings), but those layers can be swapped in and out as needed for performance, convenience, cost, etc.