Sony is effectively killing off all of their other Xperia lineup of smart devices in favor of one lineup of Xperia X products. Until at least 2018, the Xperia X line will be Sony's only mobile product venture. This means there will be no more Xperia Z, Xperia C & Xperia M.
The Xperia X series will feature three different smartphones for the three various consumer markets. At the budget-friendly end we will have the Sony Xperia XA, for the mid-range we will have the Sony Xperia X, and for the high-end the Sony Xperia X Performance will fit the bill.
Here's our dedicated section for further discussion: Sony Xperia X Series Smartphones
Source: Xperia X will be Sony’s mainstay until 2018; Xperia C and M series no more | Xperia Blog