What is the APT/SUITE field during register Apple care protection plan. What I've to fill there?

Apartment or suite number, use if needed, leave blank if not.

tt2

If you are registering an Applecare extended warranty plan, then it is normally registered to the item you want to have a warranty on. I have never needed to enter my address because they already have my address if the computer or iPhone or iPad is already registered.

So, is this the place where you are trying to register it?

http://www.apple.com/support/applecare/register/

When I click on Continue, I get this page where my name/address are already entered and all I need to do is enter the serial number for the device and registration number for the applecare. Note that the Apt/Suite area is blank:

Hmm, from the name of the field that sounds like something that should be optional.

Try contacting support in India, perhaps they can help you set it up: https://www.apple.com/in/support/applecare/contact/

tt2

I used babowa's lnk above... was already logged in by virtue of being logged in here in ASC it seems.

All the fields were populated FOR me - Including "Address 2", the APT/SUITE field with my ONLY address listed anywhere in all of AppleWorld = ADDRESS 1

Try putting your same address in BOTH fields