Usage of staff as a seniority level (“staff engineer”)

This question is about the usage of staff as an adjective denoting a specific seniority level.

Research

Some companies use the word staff as an adjective to denote a level of seniority which is above senior, but below principal. I’ve seen it applied to engineering jobs, including but not limited to software development. The general idea is that most engineers will reach the “senior engineer” level after a few years, but a “staff engineer” is someone who has not only experience but also a certain amount of influence. This isn’t necessarily a management role: it’s generally about the influence the employee has on the product, rather than about how many people they direct.

Note that this has nothing to do with the much more common meaning of “staff” as a noun, where the employees of a company together make up is staff. I’m referring to a specific meaning where staff designates a small set of employees, and is only used as an adjective — an employee can be “staff engineer”, not “*a staff”.

In my experience, this usage is well-known in some milieus but unknown in others. It doesn’t seem to be covered in dictionaries, for example:

• Merriam-Webster (lists staff as an adjective without giving a meaning)
• The Free Dictionary (gives a meaning as an adjective that encompasses all employees)
• Cambridge (no adjective)
• Collins (gives a meaning as an adjective that encompasses all employees)

The usage may have derived from military practice, where a staff officer is an officer who assists high-ranking officers directly. Some dictionaries list a similar meaning in non-military organizations, for example in Merriam-Webster 5a “the officers chiefly responsible for the internal operations of an institution or business” and 5d “the personnel who assist a director in carrying out an assigned task”. This is somewhat different from the usage I’m asking about: a “staff engineer”, in my experience, is usually somewhat autonomous and the source of new ideas and directions at a small scope, rather than passing on directives from above.

The usage is different in nursing, where “senior nurse” is short for “senior staff nurse” and is above “staff nurse”.

The meaning of “staff engineer” seems to be mysterious enough that a number of people spend some energy in attempting to define it, for example:

• What does Staff level mean at GitLab? by Charlie Ablett
• What a Senior Staff Software Engineer Actually Does by Joy Ebertz
  
• StaffEng, StaffEng FAQ

Question

How widespread is the word staff in this sense? Considering that dictionaries don’t list it, is it a recent thing? Is it regional (inasmuch as this question makes sense, as it seems to be used in technology company that tend to be multinational)?

At a guess, an average English speaker would interpret “staff engineer” as “an engineer who’s an employee (rather than a contractor)” rather than “an engineer who’s more senior than senior but not much more”. What demographic would assume the second meaning?
Is this something a typical English speaking HR employee would be familiar with, or only HR in certain fields or regions?

Do other professions (other than engineering and nursing) use staff as a seniority level, and if so how does it fit in seniority rankings?