Sony has been given Stamina Mode feature in previous Xperia devices which prevents background tasks for unauthorized apps and Sync while the feature is turned on. But it has been removed from the Marshmallow update for Xperia devices. There is already an in-built option from Google itself exists in Marshmallow named as Doze. But Doze has no customizing options. It works only when the device is completely in rest.
SO now Sony is going to bring back the Stamina feature on Marshmallow update, but Google restricted it to a third party option that it won’t be interfere with Doze in any manner. According to the Sony about the returning of Stamina Mode :
Great news. One of the most requested features is coming back to your Xperia running Marshmallow: Stamina mode. We don’t have a release schedule yet, but the feature will be definitely available for members of the Xperia Beta Program in the near future. We would like to thank you for your patience and constant feedback. Your voice is essential for the development of a better software.
Anyway, the next Beta program, which is currently running among Sony developers will feature the Stamina feature, and could be along with the next major update from Sony.