My first job in the "real world" was as a Project Scientist, so I can empathize with you. Based on my experience they probably want to use you as a "floater", a warm body that can be put onto any project that comes through the door, or alternatively to support any politically powerful principal investigator in your organization. It is like being a consultant, but with twice the headaches, half the pay and a tenth of the authority.
I have joined one research institute as Project Scientist, but there is no clarification of the job profile here.
Anytime an organization can't provide role clarity it means the organization hasn't got much of a clue around what your role should be. Or if they articulated it nobody would want the job.
from a project management perspective what is the exact role?
From a PM perspective you are one of many team members, likely more competent than some and less so than others. I would look on you as a generalist and try to keep you on pieces that don't need real speciality knowledge.
AS a human recourse manager, what is the key responsibility of such a
post in the organization? what value it adds to the project by adding
such a person to the project?
You probably have the role because there was $$ in the overall organization's budget but not in department/lab budgets. This gives a great deal of flexibility from an HR perspective, and provides a warm body at reasonable cost,