Earlier major BSD releases included 3BSD (1980), 4BSD (1980), and 4.1BSD (1981).
The standard defines two levels of conformance: POSIX conformance, which is a baseline set of interfaces required of a conforming system; and XSI Conformance, which additionally mandates a set of interfaces (the "XSI extension") which are only optional for POSIX conformance. XSI-conformant systems can be branded UNIX 03. (XSI conformance constitutes the Single UNIX Specification version 3 (SUSv3).)
The POSIX.1-2001 document is broken into four parts:
XBD: Definitions, terms and concepts, header file specifications.
XSH: Specifications of functions (i.e., system calls and library functions in actual implementations).
XCU: Specifications of commands and utilities (i.e., the area formerly described by POSIX.2).
XRAT: Informative text on the other parts of the standard.
POSIX.1-2001 is aligned with C99, so that all of the library functions standardised in C99 are also standardised in POSIX.1-1001.
Two Technical Corrigenda (minor fixes and improvements) of the original 2001 standard have occurred: TC1 in 2003 (referred to as POSIX.1-2003), and TC2 in 2004 (referred to as POSIX.1-2004).