Newb: How to Build Plugins with single tag variables

This is such a newbie question I hesitate even asking it - however, I have read through the doc on how to build a plugin, but no where can I find the docs on how y ou create a tag variable which is used insiode a plugin. They only talk about paremters.

Question: how do you create a tag variable ? Can someone send me the link to this inside EE docs or other outside tutorials ?

Many Thanks !