Confused...

["Graham Greene" <yyyyyyyyyyyyy@xxxxxxxxxxxx>]

Can "The Single UNIX Specification, Version 3" be used as a reference
for functions that are defined by POSIX? The reason I ask is because
this site:
http://standards.ieee.org/catalog/olis/posix.html
shows 4 volumes for 1003.1-2001, where volume 1 is "System Interface
(POSIX)" and for volume 2 is says "This standard is equivalent to the
Single UNIX Specification". Whats the difference? (I don't have a grand
to download them and find out.)

Also, if I wanted to write a POSIX compliant application, would it be
more likely to run a more systems today if stuck to POSIX.1 or POSIX.2?
(Again, the only thing I have access to "The Single UNIX Specification,
Version 3" on www.unix.org.

Graham Greene