The cup is a ceramic attachment that goes on the end of the TIG torch that directs the argon gas onto the weld. The cups come in many different sizes. They are numbered – the smaller the number, the smaller the opening.
Why use a different TIG cup size? Different sized cups have their pros and cons. The larger the cup, the more gas is available and the harder it is to move around. The smaller the cup, the more mobile it will be and easier it to maneuver in tight spaces.