Job requires recommendation letters from Senior Research Scientists - who is eligible to write letters? I am wondering who are treated as Senior Research Scientists?
Whether or not Assistant Professors, PostDocs, Project Scientists are treated as Senior Research Scientists? 

I am asking this because I need couple of recommendation letters to make an application for tenure track scientist position. I am asked to take these recommendations from Senior Research Scientists.
In general, I would interpret "senior researcher" as somebody who has a permanent (or potentially permanent, such as tenure track) position doing research at a research institution.  It excludes graduate students, post-docs, people with purely teaching positions, etc.

However, if you are applying for a specific position, I would add one additional criterion for what counts as a "senior researcher."  Your letter writer should themself hold a position at least roughly as advanced as the position you are applying for.  The people reviewing your application want the person writing the letter should be familiar with the hiring standards and criteria for positions at that level.  So while it may make sense for an assistant professor to write a letter for you if you are applying for an entry-level tenure track position, you would want a full professor (or equivalent) to write you a recommendation if you are applying to a senior-level tenured position.