The first version of the format was originally published on the rgc:audio website, which is the most supported by sfz related software.
The SFZ v2 standard has never been clearly set down anywhere. For the purpose of this website, anything included in the Simon Cann's Cakewalk Synthesizers is considered SFZ v2, regardless of which SFZ players it might or might not be actually implemented in.
If something works in ARIA but not in any Cakewalk products, that's considered an ARIA extension.
Cakewalk Synthesizers is not a recent book, and was not intended to be a standards document, but rather a manual for users of Cakewalk products. So, if you are developing a new SFZ player, do not feel obligated to support all opcodes, headers etc. listed on this website - instead, use your judgment.
ARIA also adds some extended MIDI CCs in addition to those already added by SFZ 2, and XML instrument banks as a way of organizing multiple SFZ instruments and configuring graphical user interfaces. See also the Plogue forum's ARIA's Custom opcodes post.