I assume you are referring to the knife sections, not the entire knife. Sections are the individual parts that are usually 3 inches wide. The knife is the assembly of sections on a bar and is approximately the length of the cutter bar.
IH made a variety of sections for different conditions. Nearly any model mower can use any of the sections. When replacing individual sections, they usually need to be replaced with sections that match the rest on that mower. Knives may interchange between mower models but need to be the proper length for the cutter bar.