Steel, stainless steel A2 or stainless steel A4.

Steel grade 6, bright or electro zinc-plated
Stainless steel min. A2-70, bright.
Stainless steel min. A4-70, bright.

Cap nuts are mainly used for decorative screw connections. The cap nut covers and protects the thread end. They are also used to protect against sharp edges on machines, equipment, fitness equipment and wherever people or objects could be injured or damaged by an exposed thread end.
The quality of the cap nuts corresponds at least to the specified value, but can also be higher.

The design type, whether thread run-out or thread undercut, is at the discretion of the manufacturer irrespective of the thread size, as per DIN 1587:2021-10.