Dear Hrsf,
What do you do and why have you raised these questions? I have a feeling that you may be a student. As John in Oz has recommended, have you searched CiteHR. If not, I suggest that you kindly do that first. I searched the web using key words from your query about merit-cum-seniority that led me to an article published somewhere. However, the citation at the end of the article was from CiteHR.
So, please search, read digest the content and tell us what you found and understood. Then, we will correct any misunderstanding that you may have. I could say what I found; but that would be giving answer on-a-plate and would not help you to become an independent learner.