For many years, some phones have integrated a small LED light that changes color to alert users whenever they get any kind of message, emails or calls. This is quite useful, as users don't need to check their phones the whole time to know what they have missed. There are alternatives that provide similar functionality in case a phone doesn't feature a notification light, such as an always-on display or some apps can be used to make the camera flash to light up when certain notifications are received. Some phones include notification lights even if they can't be used with official software.
Sony often includes LED notification lights in their Xperia phones and the company has implemented them in different ways. For example, the Xperia S lineup included a bar with LED lights near the bottom, which could be used for notifications. The whole Xperia Z lineup also feature notification lights, in most cases, the LED light looks like a small dot and it is located at the top of these phones, but some of the exceptions include the Xperia Z1 and Z1 Compact, as well as the Xperia Z2, where the LED light was located behind the earpiece, looking more like an elongated beam of light.
The company recently introduced the Xperia X lineup, which includes the Xperia X, Xperia X Performance and the Xperia XA. With these three phones, the company seems to have changed their whole strategy, as there won't be any new Xperia Z phones. The Xperia X phones will launch in the Summer, but some details that were not mentioned during the official presentation are now surfacing. It has been reported that the Xperia X includes a notification LED light as well, and it is located behind the earpiece, but this time, it is located towards the right side. The light looks quite small, but it seems bright enough to be seen from a reasonable distance. Given that the Xperia X Performance features a very similar design, it is probable that the notification light is located in that same position. It is unknown if the Xperia XA features a notification light, but this phone sports a different design with the earpiece located at the top, so the notification light could be positioned somewhere else.