The America Invents Act of 2011 (AIA) replaced the intricate “first-to-invent” prior art definitions with simplified definitions that now govern virtually all USPTO prosecution. However, the first-to-invent prior art definitions still govern when assessing the validity of pre-AIA patents, those based on US applications filed before 16 March 2013.

Reliably assessing the validity of such pre-AIA patents can be challenging, for at least the reason that much first-to-invent prior art can be disqualified based on an earlier invention. A proper appreciation of the full scope of the first-to-invent definitions, coupled with inspection of certain public (and, if available, non-public) information, can render validity assessments of pre-AIA patents more reliable.

David Loretto gives the full explanation in this article.