General terms, concepts, and interfaces common to all volumes of POSIX.1-2017, including utility conventions and C-language header definitions, are included in the Base Definitions volume of POSIX.1-2017.
Definitions for system service functions and subroutines, language-specific system services for the C programming language, function issues, including portability, error handling, and error recovery, are included in the System Interfaces volume of POSIX.1-2017.
Definitions for a standard source code-level interface to command interpretation services (a “shell”) and common utility programs for application programs are included in the Shell and Utilities volume of POSIX.1-2017.
Extended rationale that did not fit well into the rest of the document structure, containing historical information concerning the contents of POSIX.1-2017 and why features were included or discarded by the standard developers, is included in the Rationale (Informative) volume of POSIX.1-2017.