What is a Research Scientist position? In an advert, I read:
"[University U] is inviting applications for a multi-year fully funded Post-Doctoral Associate position. Highly qualified candidates may also be considered for a Research Scientist position."

What is a Research Scientist compare to a Post-Doctoral Associate position in an USA Academic like environment ?
Every university is different. However, my university (UT Austin) has, roughly, three research-only track positions: Research Associate, Research Scientist, and Research Professor. None of them are tenure-track or require teaching, and they all are roughly equivalent to the tenure-track positions: Assistant Professor, Associate Professor, Professor. Research Scientist has roughly the same promotion requirements as Associate Professor but only requires 3 outside letters rather than 8, and is a little lighter than the research and service requirements to the best of my knowledge. Both tracks can be proceeded by a postdoc position, but often do not. I started at UT as a Research Associate straight out of my PhD program and was later promoted to Research Scientist.

These positions are offered to give the university a place for professional researchers in non-teaching roles to work without the constraints or requirements of a teaching job. We have lots of them only mostly grant/soft money at the computing center where I work.