There are eight attributes to magic: Fire, Water, Earth, Wind, Lightning, Ice, Light and Darkness.
Unattributed means having none of these attributes. In other words, someone unattributed could not cast magic of other attributes.
Originally one is supposed to only have one attribute. Of course, there are exceptions and talented individuals possess more than one. But it is rare for anyone to have three or even four attributes.
Moreover, the light attribute is basically only used by Pheoms (with the exception of heroes).