Milling In Drilling Operations Guidelines For Oil & Gas
Milling is the process of steel removal. Such milling may be mandatory in a lot of cases while drilling oil and gas wells after trials of fishing operations. It will also result in a removal of the steel obstruction in the wellbore and continue drilling to the well target.

Milling Applications In Drilling:

Milling is not designed only for the fish in the hole (check
https://www.drillingmanual.com/milling-in-drilling-oil-gas/