This is my first time on a flight. So pardon me for the question. My luggage weighs more than the specified limit. I read elsewhere that in order to avoid excess baggage fee, one can send the luggage through 'unaccompanied baggage'. But there wasn't any more information regarding the unaccompanied baggage. So how do I use the unaccompanied baggage facility? And what are the things I should know when I use the service?
Unaccompanied baggage is freight or cargo.
For the formalities you need to go to the excess baggage counter at the airport when you check in.
It is an alternative to having to pay excess baggage fees. The unaccompanied part means that your luggage doesn't travel on the same flight as you do; and the rates are considerably lower than the normal cargo rates.
You didn't mention the airline, but most airlines have a page describing the service and restrictions. Here are pages from Air Newzealand, Air India and KLM to give you an idea of what this service entails.