It varies wildly, but at the end of the day its about skills being rare and valuable. People with rare skills, who can prove they have those skills, and can consistently perform complex/difficult tasks and be reliable are what I would consider “top tier”
Asking for some specific defined standard is being pedantic. The standard is being capable of things that others are not, and that’s true across any industry. Each one will have its own measurements (certifications, work portfolios, references, etc.) by which those are defined.