There were several backs that used Polaroid, instant pack films, and today, these backs will accept two (2) different sizes of Fuji pack films.
The smaller 3.25 X 4.25 size pack, and the larger 4 X 5 size pack.

Polaroid 405 series backs were produced, (and sometimes, third-party modified en-masse), for 35mm cameras, MF cameras, and LF cameras.
The majority of these Polaroid backs are for MF cameras, but some Polaroid 405 backs were made for 4 X 5, LF cameras.
Fuji also made a small back, the PA-145, that fits LF cameras, as well as Fuji's MF cameras.

The Polaroid 405/Fuji PA-145 backs accept Fuji FP100c, FP100b, and FP3000b, 3.25 X 4.25, pack film, 10 sheets per pack.
It does appear that only the FP100c and FP3000b are still in production in this smaller size, though shelves are still stocked with the FP100b.

The larger 4 X 5 backs are IIRC, exclusively for LF cameras only. These backs are the Polaroid 550 and the Fuji PA-45.
Pack film still available from Fuji for these larger backs, again with 10 sheets per pack, are FP10045c and FP300045b,
though the black & white version in this larger size has been discontinued.

I have seen 405 backs for Nikon 35mm cameras, Mamiya 645's, RB's and RZ's, MF Fuji's, Mamiya and/or Polaroid-Universal/Press type cameras, and Cambo Passport cameras. There are many others, but I have not seen/handled them personally.

IIRC, Fuji pack films were not officially allowed/imported to North America until after Polaroid's first bankruptcy in 2002.
In my opinion, Fuji pack film, was, and has always been, superior to Polaroid's pack film.