Pico BTX
BTX
It is BTX
The three versions are; Micro, Nano and Pico
atx,nlx,btx
The three form factors for desktop system boards are: ATX, and BTX and NLX
Atx, btx, nlx
ATX
Full-size tower (BTX)
The BTX Form factor design focuses on reducing heat with better airflow and improved fans and coolers.
The main difference between ATX and BTX, is that BTX rearranges components on the ATX board to improve thermodynamics. Components changed that reduced the heat production within a PC, negating the need for the switch to the BTX board.
ATX is the main form factor of today. Other form factors are basically just variations of ATX. Example: BTX & uATX
I'd say ATX was the most popular form factor. Assuming microATX and the like is included in that. BTX also existed at that time however, it never really caught on