Depending on the format....

.tub, for example, would be plug-ins and .png would be 2D.

Actually, Tubes shouldn't be in any section until they add the 2D section they're talking about adding. Tubes are not "Just for Poser" nor are they Plugin's which is a term for Software code that adds functionality to a program.

The closest section they fit in for now would be either Textures or Materials as they're usually used as textures on a square prop inside a 3D program in place of an actual mesh of an item.

Actually, I disagree....tubes, as in PSP tubes (.tub, .PSPTube), are used in Paint Shop Pro, they are to be "plugged in" (installed) into the correct folder to be used in that image editing program to function as a PSP tube.

.PNGs can be used/opened by any image editing program (PS, CS, PSP, Gimp, etc.) for 2D purposes such as website designs, signatures, printable cards, and various other decorative ways. 

Most look at these "tubes" for 2D purposes, especially figure "tubes".

It's not a matter of something you can disagree on Kimber. By Definition, a Plugin is a piece of Software code for enhancing a particular program's capabilities. Period. Tubes do NOT fit the definition by any stretch of the imagination. Tubes are content. Not Code.

From Wikipedia, the free encyclopedia

A plugin (plug-in, addin, add-in, addon or add-on) is a computer program that interacts with a host application (a web browser or an email client, for example) to provide a certain, usually very specific, function "on demand". Applications support plugins for many reasons. Some of the main reasons include: enabling third-party developers to create capabilities to extend an application, reducing the size of an application, and separating source code from the an application because of incompatible software licenses.

For CG related Plugin's, most items are in some form of python script or other scripting language language. As far as PSP goes, Scripts such as the ones found Here could be considered plugins.

It is a miracle that curiosity survives formal education. - Albert Einstein

This little jab at my expense --> "Well if we went by Kimber's definition..." was not called for.

If "we" went by YOUR definition of what a "tube" was...well, I'll leave the rest of my comment to your imagination. 

Could you please put a little more courtesy into your replies &  comments so they don't get perceived as personal slights towards others?