uv-mapping

How are UV coordinates measured?

I've got a 3D modeling program exporting a UV mapped 3D model. However the UV coordinates seem to be invalid or using a system I don't understand. I need help in understanding how UV coordinates map to the pixels of the bitmap (X, Y) I've got a couple of textures in the range of 150x100 to 200x600. Here are some extracts: vt 1.040...

Why are these UV mapping coordinates represented in 0-1 and pixels? Shouldn't they use only the 0-1 system?

Well, I've got many UV coordinates of a 3D model clearly in the range 0-1 and many coords in seemingly pixel values in the range 100-300. How do I know which one to interpret the coordinates as? Is there a standard or system for this? I notice that each 3d meshes in the exported file may use either system. Is this normal procedure, and...