This is the version code following the dot. Thus version 5.0 of MSIE would yield the value 0. At least, that is the theory. MSIE version 4.5 for Macintosh also yields the value 0 for this property.
Be wary of the value returned by this property on MSIE. It may not be consistent with what you expect. Bug fixes and patches are normally not reflected in this value although you can parse appVersion or userAgent for further information and in the MSIE browser you can also get access script engine version and build number values.