Principal Research Scientist vs. Senior Research Scientist?

According to the MIT EECS pages, there are two tracks:

• Academic Research Staff. These are mostly shorter term (such as a Postdoctoral Fellow), and are funded externally (by grants and similar).
• Sponsored Research Staff. These are longer term appointments which are funded by the department.

Senior Research Scientist is an Academic Research Staff position, because while it is long-term, it is also a position for which the researcher is expected to procure their own funding.

From 5.3, the SRS's funding:

Appointees to this rank generally are expected to raise funds to conduct research and cover their own salaries.

Principal Research Scientist is a Sponsored Research Staff position, and is funded by the department (though I'm sure they can get their own funding also, their salary is paid by the department).

From 5.2, the PRS's funding:

Funding responsibility resides with the department, laboratory, or center that is the locus of the appointment.

From 5.1 on the difference:

Two types of research appointments are described in the following sections: long-term appointments, which are classified as sponsored research staff appointments, with the exception of the Senior Research Scientist/Associate/ Engineer position; and short-term appointments, which are, in general, classified as academic research appointments.

The purpose of the campus research appointment structure is to create professional sponsored research staff categories with employee benefits and privileges in order to encourage long-term research careers at MIT. The concept of long-term professional careers, independent of classroom teaching and supported entirely from research grants, is not a new one for the American research university. This mode of research, with professionals working in departments, laboratories, and centers, is an important mechanism for universities to maintain research viability, enter new fields, and bring researchers to the campus.

It seems evident that Senior RS is basically the same position as PRS, but not funded by the department. In some cases, the PRS could earn more, because (for example) a NIH-funded SRS would be limited by NIH regulations, while a PRS who is funded by the department is only limited by whatever MIT chooses to pay them.

For scientists,there are "Research Scientist", "Principal" and "Senior". The latter two can be PIs. The later two also have short term tenure meaning the university will cover their pay for a short term should their funding lapse. I believe for Senior it is one year. Senior are equivalent of full professors in rank and help set the research direction of the whole lab or group. Principals can seek and run their own grants and may or may not have some departmental funding if the dept has large support grants.

Senior is definitely a bigger deal than Principal. There is a big salary step to senior. At MIT Senior is limited to 10% of faculty and principal to 15%. Scientists don't have to teach or have grad students and have no "tenure clock". you will typically find them at larger research groups and labs that have dozens if not over a hundred staff and students.