Format used to refer to what kind of camera you used — since each had a native shape. A 4x5 camera used 4 by 5 inch film holders. 35mm was a format, the shape of which was in a ratio of 2 to 3. Hasselblad was famous for its square shape. Today, digital cameras still have sensors that come in ratios of 2:3, 16x9, or square (go Hasselblad!) but since most images are processed in computer, it is very common to crop to a new shape at that time — or to combine multiple shots into panoramas.
Notice how the various shapes don't "play" well together - they create awkward spaces between them — which is one reason I chose squares for this project: they stack well.
Bottom Line: The format you shoot with will favor some subject matter over others.
All of these "formats" were cropped from a single photo: 24mm, f9.5, 1/180