oh right, you saw someone use extract and then gave each piece different materials? or they just used extract to create different 'UV shells'?
extract, depending on the settings, will separate faces from the mesh and either keep them as part of the mesh or separate them.
i've never seen anyone use extract to create different UV shells, but no, that isn't a must when laying out UVs.
that's a "Ch" pronounced as a "K"
Computer skills I should have:
Objective C, C#, Java, MEL. Python, C++, XML, JavaScript, XSLT, HTML, SQL, CSS, FXScript, Clips, SOAR, ActionScript, OpenGL, DirectX
Maya, XSI, Photoshop, AfterEffects, Motion, Illustrator, Flash, Swift3D