In fact, they had the same technical characteristics except from built-in floppy drive (hence the f of STf).

The 3.5" floppy disk drive has been integrated with the power supply into the computer.
The early first versions of the Atari 520 STf had a RAM based Operating System (they have a 32 KB ROM),
this ROM will be quiclky replaced by a 192 KB ROM which holds all the operating system (called TOS 1.0).

An enhanced version of the Atari 520 STf was launched a few months later :
the Atari 1040 STf to replace the Atari 520 :
It also had the same characteristics as the 520 STf except its memory (1 MB instead of the 512 KB)
and the floppy disk drive : it used 3.5" double side disks (720 KB).
Not long after the launch of the 1040 STf, the 520 STf would be "unofficially" equipped with 720 KB floppy disk drives.
There was also a STfM model with a built-in floppy (the f) and an RF Modulator (the M).