This refers to an attribute that a source can be given go prevent clashing with another character's source, usually given to open-ended or otherwise overpowered sources. For example, if there is character with "Nintendo Music" and another character with "Doppelganger Themes", then adding NO OVERLAP to one of the sources means songs that fit in both types of songs will only go to the source with NO OVERLAP. The only time this doesn't apply is on Duel Tracks, in which songs which fit in both character's sourcelists get listed as going under both characters.