Custom Weapon Types

I've been looking all day for specific information about creating different types of weapon damage, however I can't find anything definitive that actually explains how I would go about doing this. In terms of actually creating the damage I think I have a few ideas, but I'm clueless on how it would display said damage on the tooltip nor am I particularly certain about it.

I've tried looking at a number of examples, but it seems like this has only been implemented in closed-source mods....

Custom Weapon Types