Some Elna machines require cams to make fancy stitches. Each is identified by a number and picture of the pattern it produces. The top picture on this page shows where cams are inserted. The pattern the cam on the right creates can be seen in the photo.
Elna Supermatic Cam Assortment
Some discs have a single cam and others have two cams depending on complexity of the pattern.
Below are two images showing Single Discs and Double Discs patterns.