POSIX stands for "Portable Operating System Interface" Portable Operation System Interface. The main reason is that the UNIX system developed too fast in the early days. The establishment group headed by SYS V and the academic group headed by BSD each made a lot of new things, and there were more and more competition and incompatibility between them. Both ends of the mouse, at a loss. So some good people came out to unify the rivers and lakes, called all the hills to sit down and talk, unified the mess of each family, and proposed a standard API that all parties could accept and support, basically a combination of SYSV and BSD. As long as the program is written according to this API standard, it can theoretically be compiled and run on various operating systems and hardware platforms.
The IX abbreviations on the tail are to express the kinship with UNIX. The name is still from Stallman's beard.
As a new operating system developed from scratch, Linux has wisely chosen to use POSIX as the API design standard in order to obtain as much application software support as possible after its gradual development.
The US Federal Information Processing Standard stipulates that products purchased by federal government agencies must comply with the POSIX standard. Windows NT has made a fool of it. In fact, the early version cannot be used at all.