This report is useful in seeing what versions need to be supported by your site. This is important since sites can function differently or not at all under different script versions.
Terms Used on Report:
Also referred to as a 'Unique Visitor'. By default, this value represents a visitor that comes to your site for the first time in a day. All subsequest visits during the day by the visitor are not considered unique.
The definition of a visitor can be customized. You can set a custom period of inactivity that will cause the next access of the visitor to be considered a new visit. The visit definition screen can be found on the 'Setup' menu.
This differs from a page view or hit in that a single visitor can have one visit per day but many page views during a visit.