Why processes on the internet are not required to use TCP or UDP?


Here is an excerpt from Andrew S. Tanenbaum, Computer Networks, 5th edition, Chapter 5 (The Network layer), Page 455:

[Fifth, processes on the Internet are not required to use TCP or UDP. If a user on machine A decides to use some new transport protocol to talk to a user on machine B (for example, for a multimedia application), introduction of a NAT box will cause the application to fail because the NAT box will not be able to locate the TCP Source port correctly.]

My question is in this context what does processes means? Is it a process of data exchange between routers? And why processes on the internet are not required to use TCP or UDP? Isn’t it apart of IP protocol and thus it will be used on the internet?