Program Differences- P-8 vs KC-46
If I am not mistaken,the P-8 is being procured strictly as a military aircraft, certified to military specifications and airworthiness requirements- derived from a civil airplane design, but neither seeking civil airworthiness certification nor achieving it.
On the other hand, the KC-46 starts as a new FAA-approved 767-2c, modified by STC with installation of numerous military subsystems and finally provisioned with certain military systems and functions that meet only military airworthiness requirements.
If that sounds "simple", it's not and it has been made all the more complex by an unending series of revisions of what belongs in each "bin" and a complex sequence of assembly of the pieces. All with their share of controversy and misunderstandings.
Program management at its finest.