Some devices belonging to the Xperia series in both the high and mid-range segment will skip the last iteration of Lollipop that brings Android 5.1.1, and instead directly receive the Marshmallow flavour. At least Sony's software support pages suggest so.
While Sony has already made the Android 5.1.1 firmware update available starting with Xperia Z3, Z3 Compact and Z2, and then rolled out the update to Z1, Z1 Compact, Z Ultra, and the entire Z series such as ZR, ZL and Tablet Z, the devices released in H1 2015 are still sitting on Android version 5.0. These devices are Xperia Z3+ (which is also known as Z4), Z4 Tablet, C4, C5 Ultra, M4 Aqua and M5.
Earlier, it was speculated that Sony might bring the latest Lollipop treatment for the above set of devices followed by the launch of Xperia Z5, which did not materialise. But now Sony's official software page for each of these devices suggests that they will receive the software bump directly to Marshmallow. The software pages note the launched Android version for them as 5.0 and the next version as Android Marshmallow, which is a good indication for the Marshmallow upgrade.
In early October, Sony revealed its Android Marshmallow rollout plan and roadmap, which included the above set of Xperia devices. Sony did not mention the rollout schedule but the Xperiablog claims that the owners of these devices may expect it before the end of 2015.
"We've been working hard to bring this major update to as many of our devices as possible, as quickly as we can. We're really excited about the new Sony features & functionality and the improvements we've made to the user experience – and we'll be sharing more soon... so stay tuned to the blog and SonyMobileNews for all the latest news on timings and features," Sony said while detailing its plan for the Marshmallow upgrade.