Welding rods come in different diameters, lengths, and gauges. Depending on what you're welding, you may want to choose a rod with a specific diameter or gauge. P91 material welding rod comes in different types, lengths, and diameters to suit your welding needs. 

Here are the most common types of welding rods and what they're used for:

TIG Welding Rods: Used for Tig welding, these rods have a very fine wire electrode that helps produce a smooth weld. They're also typically long and thin, making them ideal for working with small pieces or tight spaces.

Gas Shielded Welding Rods: Used for welding with gas-shielded electrodes, these rods have a thicker wire electrode that helps protect the weld area from heat damage. They're also shorter and wider than TIG welding rods, making them better suited for larger pieces or more complex designs.

MIG Welding Rods: Similar to TIG welding rods, MIG welding rods have a fine wire electrode that produces a smooth weld. However, they use an electric arc instead of a gas shield to create the weld. 

This makes MIG welding less damaging to the fabric of the object being welded, making it better suited for delicate jobs. It is essential that you select the correct welding rod for the job at hand.