It's not a plug-in. It's just one process involved in creating 3D models. The modeler has to layout the visible faces for his 3D mesh, and scale and postition them to fit on a single 2D texture. And as you questioned, no; in most circumstances, the "hands" and etc, don't need to be in any particular area of the texture, so long as none of the parts overlap.
Quote: "UV mapping is a 3D modeling process of making a 2D image representing a 3D model. The map transforms the 3D object onto an image known as a texture. In contrast to "X", "Y" and "Z", which are the coordinates for the original 3D object in the modeling space, "U" and "V" are the coordinates of the transformed object. This creates the effect of painting the image onto the surface of the 3D object."
Source:
Wikipedia
Kravenwolf
