Is Linux considered XSI compliant or largely so?

From APUE > The Single UNIX Specification, a superset of the POSIX.1 standard, > specifies additional interfaces that extend the functionality provided > by the POSIX.1 specification. POSIX.1 is equivalent to the Base > Specifications portion of the Single UNIX Specification. > > The X/Open System Interfaces (XSI) option in POSIX.1 describes > optional interfaces and defines which optional portions of > POSIX.1 must be supported for an implementation to be deemed > XSI conforming. These include file synchronization, thread stack > address and size attributes, thread process-shared synchronization, > and the > _XOPEN_UNIX symbolic constant (marked ‘‘SUS mandatory’’ in Figure 2.5). Only XSI-conforming implementations can be called UNIX systems. Is it correct that SUS consists exactly of POSIX and XSI? Is it correct that Linux (or Ubuntu, Debian in particular) is POSIX compliant? Is Linux (or Ubuntu, Debian in particular) considered XSI compliant or largely so? I ask this because then I will know whether the parts in APUE labelled for XSI apply to Linux (or Ubuntu, Debian in particular). I am mainly interested in API, so does that mean Linux kernel suffices?