"Not bad, eh? Merely an optical illusion. But it lasts 8.6 hours and should prevent the locals from becoming too suspicious."
-- Emmett Brown after demonstrating the device for the first time.
A clothing converter was a device used by the Brown family to blend in when they visited a different time period.
The device worked by producing an optical illusion, which would last for 8.6 hours before disappearing in a flash of light. In order to function properly, the device required 'clothing conversion coordinates' to be entered.
On one occasion, Doc Brown held up an existing photograph of 19th century people, wearing the clothes that would later be used as their own disguises. It is therefore likely that the clothing converter sources its clothes from existing images (or possibly paintings if used in an era before the invention of the camera).