Dear Tharun,
You need to refer Manufactured product definition provided in Central Excise Act.
For Different categories difference Codes are provided accordingly the products classified.
In your case If a Pen for ultimate sales requires Pen Cap to cover Tip of pen and even "Inked Refill", bringing all together it becomes final Finished Goods "A Pen" for Sales, All though Individually all items can be Sold (Generally Job Worked for Outsiders) as Finished Goods too.
To Add, Individually this are called Finished Goods, But for Ultimate purpose (here "Pen" with Cap) they are Semi-finished Goods, U can not sell Pen without Refill or other necessary accessory which brings its Identity as Pen.
But you can Sell "Refill" individually as finished goods.
Hope it is clear.