A system only SBF will only replace your file system, which is used to.upgrade your phones file system to the latest update. In this case, .340.
A full SBF will take your phone back to and or upgrade your phone to. 340. This will bring your to an "Out of the Box" state. Which will upgrade your system, baseband, kernel, and bootloader. And you will have to reactivate with *228 option 1.