Slotted/Castle nuts are pre-drilled, they can be used without a pilot hole. These nuts are slotted, allowing them to be installed and removed using screw drivers. It is used primarily as safety nuts, these nuts are slotted to receive a cotter pin or wire which passes through a drilled hole in the bolt and locks it. In assembly the nuts are torqued to preload value, then the nut position is adjusted to line up the slots with the hole in the bolt.
When a castle nut (or any radially slotted nut) is applied to a threaded section, it is tightened so that its slot is aligned with a predrilled hole in the bolt or threaded rod, into which a cotter pin, split pin, a lock wire or a spring pin is typically inserted to stop the nut from rotating.
Castellated, slotted and castle nuts are commonly used on axles and are even called axle nuts in some applications. Sometimes they are used on trailers too. They are also used as hub nuts that hold the driveshaft within the wheel hub assembly.
At CE it is available in stainless steel, carbon steel, and alloy steel to bear more wear and tear while also making it more corrosion resistant.