The payload section of a TCP segment contains the actual data being transmitted, which includes application data such as web pages, emails, and other types of files. The size of this segment can vary depending on the amount of data being sent between two computers. Additionally, some protocols incorporate data into the payload section that is necessary for communication between two computers, such as sequence numbers and other flags. The payload section can also include additional error-checking codes to make sure data is received correctly. The length of the data section is not specified in the segment header; it can be calculated by subtracting the combined length of the segment header and IP header from the total IP datagram length specified in the IP header.