Sony Flash Tool is a flashing software for updating, unlocking, rooting, and custom kernel Sony smartphone operating systems using FTF firmware. Created by Androxyde based on the command line written by Bin4ry (Andreas Makris) which is given an interface for flashing sin files. This program was originally made to flash sin files downloaded by SEUS (Sony Ericsson Update Service), PCC (PC Companion), or XperiFirm. Flashtool is a S1 flashing software that works for all Sony phones from X10 to Xperia Z Ultra. They all use the S1 protocol for flashing firmwares.
From time to time, sin files have been bundled into what is now well known FTF (Flash Tool Firmwares) and more features have been implemented. But flashing firmwares is still the core of Flashtool (that is updated at least to follow Sony improvements around sin files) and the reason of its name. Flashtool can also easily unlock the bootloader of the phone using the BLU icon as far as the bootloader of your phone is unlockable.
The flashing feature as well as bootloader unlock feature are available whatever the phone is recognized or not by the application. What is only mandatory for flashing is to own the FTF file according to the device you want to flash it on.