The Shimano RD-6870-SS is their short cage rear derailleur for the 6870 series. It supports a max front difference of 16T, a low sprocket minimum of 23T and a low sprocket maximum of 28T. The top sprocket maximum is 14T and the minimum is 11T. It has a total capacity of 33T.
It also features Shimano's Crash Protection Mode - this prevents shifting after a crash to make sure the rear derailleur is damaged any further. To restore shifting functionality have a look at our FAQ I had a small crash and my rear derailleur stopped working. What happened?