To be more precise : FlashTool need to have full read/write access to all the various USB devices that the Xperia X will present to the system.

sudo is a quick shortcut to get the things done.
(But a bit dangerous in theory, because Flashtool will have full read/write access on anything...)

The long way is to add a bunch of rules in some "/etc/udev/rules.d/somthing.rules" file.

You need to add rules for every different DEVID that the device will exhibit (the Xperia X tend to have a multiple personality disorder and exhibit different IDs in each mode) :

here "users" is a group where I have read/write access.
0fce:e1e0 is the USB ID that the Xepria X present when in normal android mode.

You can see the Vendor/Product ID by doing "lsusb".

You'll need to add entries for the flashmodes (Volume Down + turn on, Volume Up + turn on, etc.). Each time, reboot the phone in a mode, do "lsusb" to see the USB ID, add a new rule in the udev rules.

After that FlashTool will have all the rights it needs, but only those.

Somebody who is not in a hurry should collect all the IDs (I mean, beside 0fce:e1e0) and add them to the tutorial.
(Together with a nice "90-xperia.rules" ready to be dropped in etc).