Gayyam and Kangkan : from a source of mine this is what i came to know about Grade - B is that it is equivalent to a Manager's Post , a person can be posted or assigned any of the various depts. of RBI like currency,supervision of banks,et al..
Also,afaik Grade B is the highest post in which a non-eco background person can join RBI since other higher posts have a pre-requisite of an Eco degree(PG).
Above all,RBI is a class apart to work with.
The link can be an addendum :
http://www.rbi.org.in/scripts/BS_SpeechesView.aspx?Id=666 Hows' ISI going Gayyam??