Actual = length of Seattube from center of bottom bracket to center of top tube's intersection with seattube's centerline.

Virtual = Length of seattube from center of bottom bracket to center of an imaginary (virtual) horizontal top tube where it would intersect with the Seattube.

Freefall's first post is a good summarization.

If you get a really old MTB, it'll have a horizontal top tube, so only one number to deal with.