Sony Mobile must be closer to releasing the official Android 6.0 Marshmallow as it has released an “Xperia Beta Program” to iron out any bugs before it is launched to the public. This beta program is not to be confused with the ‘Concept for Android’ program which remains a separate branch of software.
The Marshmallow being tested here is a pre-release version of the official Sony software that will shortly be publicly hit handsets. It is live for the Xperia Z2 (D6503), Z3 (D6603) and Z3 Compact (D5803) across only a few regions (Italy, Spain and Netherlands only).
This is how Sony describes it in a Google+ post in the Sony Concept community:
Xperia Beta Program – Quality beta test of official software
Concept for Android – Experimental consumer driven software
If you are a Concept user and wondering when the next update is coming, the team has said that hopefully a new release should be expected within a couple of weeks.