Job Description - Query

I have recently joined a new Organization and currently we are working on the job descriptions in the company. It is an IT company. As far as I know JD's are made for all positions in any Organization. But my manager has told me to group jobs having similar functions/responsibilities and create just 1 JD for te grouped jobs. For Eg: if there is a Junior and senior data administrator , create just one JD for an Administrator. Again if there are different Officers - like HR Officer,Finance Officer, IT Officer... group them under one job of Officer and prepare JD just for role "Officer". I want to know whether this is valid? I was under the impression JD's are to be prepared for all jobs.
Pls help.

Well, its been since Mar 2012, no answer
Yes, you are correct. JD needs to be prepared across vertical after careful "Job Analysis".
But typically in Middle East, depending upon the company requirment and functioning, they just make the JD based on their immediate open Vacant Position based on business requirment.
I can see that your Manager is looking at making "General JD" for positions vacan or filled. You may advise him to have diffierent JD for various position, as it will make sense in hierarchical structure Levels. (example Jr.HR and Sr. HR and Asst. HR (Whats difference and Job responsibility and KRA between all 3, if you have same JD).
Wish you all the Best.
Ukmitra