Roll Forming v. Flat Forming
Roll Forming is a continuous metal forming process taking sheet, strip, or coiled stock and bending it between successive pairs of rolls that increasingly bends it until the desired shape is completed adding both strength and rigidity to lightweight materials.
Flat Forming is the same process, but is done without the rollers. Instead, it utilizes a "pinching" action and forms the stock in a non-continuous process.
In the case of a roof seamer, roll forming occurs when the roof seamer is moving along a rib. The unseamed panel is fed through the front of the machine, passed through a set of forming rollers and a finished seam is produced. It is the gradual process of each set of tooling moving the metal a little at a time until the desired shape is achieved that makes a roof seamer a "roll forming machine".
In contrast, when you use a hand crimper to start the roof seamer onto the panel, this is flat forming. You physically take the rib and "pinch" it in one area at a time.
If you have ever been around a standing seam metal roof before, take note of the beginning of each seam. Although it will be difficult to see, you will be able to distinguish the small differences between the two forming processes.