Question about the IKEA MALM bed frame, assembly instructions

Good day!

I have a question about the assembly instructions for the Malm bed frame. I do not understand what is meant on page 12 with the two upper illustrations. It concerns these plastic plugs (102267). I do not understand what is meant by the crossed-out illustration and the non-crossed-out one, and when these plugs should be used and when not.

If you have a fixed slatted frame, you do not need these plastic caps. The reason is, if you have a slatted frame without a fixed frame, you need to secure it so that it is firmly tensioned. Otherwise, it will shift and the frame will fall down and can hurt....