Each of those messages holds the data in an Integer Variable that you have created for that purpose, and which you have "associated" with that pushbutton control.

During the design process, you used the Visual Page Designer to create that variable, giving it some meaningful name like "D1" and a meaningful comment like "LED D1 connected to RJ7".  TCPmaker will also create a short Id string for this variable, which is how the variable is actually identified in the messages sent between the PC and the micro.

In the code it generates for your device, TCPmaker will declare variables with the names and types that you created in the Visual Page Page Designer (like D1), along with places to send and receive this variable.