[NTG-context] \definesomething?

Hello, In the ConTeXt manuales there is a reference to \definesomething as a way to create new commands(?)/variables(?). The truth is that I haven't bee able to truly understand if this is a generic example or I can define my own commands with this and how the syntax would be used. I would appreciate if anyone can clarify this to me. Thanks, Pepe