The Rolleiflex T comes in type 1 (grey leatherette) and type 2 (black leatherette) versions.
Type 1 is able to make images in 6x6cm format as standard, or with the additional Rollei kit in 4x5.5cm (not 6x45) or 4x4cm (superslide) formats.
Type 2 (the last commercial version known as the white face) can do the above but also shoot 24x36mm on 35mm film using the Rolleikin kit. This effectively turns the 75mm Tessar into a head-and-shoulders portrait lens.
The 75mm Tessar in the 'T' used Lanthanum glass in the lens elements as an improvement on previous Zeiss Tessar designs.
I have a Type 1 'T' and bought it specifically to make 4x5.5cm as well as 6x6cm images. The difference in quality between 4x5.5cm and 6x45cm is negligible in enlargements.
It's a fine camera and I really like the image quality from the Tessar lens.
Hope you enjoy your camera.