Sony has started beta testing a Marshmallow version for select users of the Xperia Z3, Z3 Compact and Z2. This particular Marshmallow beta is a different take on the regular Marshmallow concept. Unlike a traditional beta version that is a pre-release test version that eventually hits all handsets, this is a form of experimental software to test out new ideas.
The version being seeded is the latest Android 6.0.1 with build 23.5.A.0.486 but is only available for devices participating in the Xperia Beta programme. Sony had announced a few months ago that it would roll out a beta programme limited to select European countries like Sweden, Denmark, Norway, Finland, Iceland, Estonia, Latvia, Lithuania, United Kingdom, Ireland, and Germany.
Sony has already released the Android 6.0 to its newer flagship devices such as the Xperia Z3+ and Z5 series with the latest February 2016 security patch. The beta version announced, however, has an outdated security patch dating back to December 2015. From the looks of it, it seems that the Xperia Beta will be used to directly communicate with Sony's developers to refine the Marshmallow update for all devices in future.
Apart from the regular Marshmallow features from Google, the Xperia Z2/Z3 owners in the beta programme report seeing a new Camera 2.0.0 user interface like the Xperia Z5 models. Changes in the new firmware also include a new launcher complete with search in the app drawer, the removal of STAMINA mode and no option for adoptable storage.