P5 is the newest set of markup guidelines issued by the TEI consortium. Currently, the so-called Blue Book is the only document being encoded by Whitman Archive staff using the P5 standard. All other documents are encoded using P4, though a migration from P4 to P5 is planned for the near future.
For examples of differences between P4 and P5 that directly affect the Archive's encoding practices, see this wiki, which documents encoding decisions regarding the Blue Book.
(entry last revised: January 2009)