It is not a part of the packet sent over the network.
It is only a help for explaining which parts of the IP header are included into TCP header checksum calculation.
Let's start with the fields that will be found in a Layer 4 TCP header: Note that a TCP header is also composed of 32-bit words; like an IP header, the default size is 20 bytes if the option field is not used.
Let's summarize the fields that are available in a TCP header: lists which applications use which port numbers.
Sequence Number TCP is responsible for ensuring that all IP packets sent are actually received.