These plugs are used for FCS fins, also known sometimes as dual tab fins.
If you want a thruster setup you get 1 center plug set and 1 rail plug set. For a quad you get 2 rail plug sets. For a 5-fin setup you get 1 center plug set and 2 rail plug sets. The center plugs have the letter C moulded into them. The side plugs have the letter R moulded into them together with an arrow pointing to the rail of the board. A common mistake is that people think the R indicates the right side and a L would indicate the left side, this is false.

Small color accents might be used to indicate the difference between 0, 3, 5, and 9 degrees. These colors are often not visible on the product photos. All plugs come WITHOUT screws.