Based on Android 6.0.1, the Marshmallow update for the above handsets packs in goodies like a new and improved Camera app, an updated launcher app, and February Android security patch level. All the Marshmallow-y features like App Standby, Doze and improved system performance are also a part of the package.
Due to some underlying changes made by Google, the update removes the STAMINA mode feature from the handsets. Sony, however, plans on reintroducing this feature with a future software update.
While the update is only seeding to select variants of the Xperia Z2 (D6503), Xperia Z3 (D6603), and Xperia Z3 Compact (D5803), it should only be a matter of time before Sony expands the rollout to other variants of the handsets.
Sony had rolled out the Marshmallow update for the Xperia Z5 and a couple of its other flagship devices from 2015 in early March.[Via Xperia Blog]