[XeTeX] openType and xetex

[maxwell]

On 2013-12-03 16:43, Khaled Hosny wrote:
> http://tug.org/pipermail/xetex/2013-March/024118.html

Is there an explanation of xetex's version numbering somewhere?  The 
copy I'm running gives the ff. version #:
    XeTeX 3.1415926-2.5-0.9999.3-2013060708 (TeX Live 2013)
You'll perhaps pardon me if I don't understand that...  I mean this
    http://tug.org/pipermail/xetex/2007-March/006057.html
is one thing, but what is the relative meaning of
    3.1415926  (I recognize this number, but?)
    2.5        (?)
    0.9999.3   (third, er fourth release of 0.9999)
    2013060708 (date)
Anyway, if the 0.9999.3 part is any indication, xetex is now using 
Harfbuzz, correct?

    Mike Maxwell
    The gentle giants of Harfang salute you.