Simplify the following text for a Grade 6 native English-speaking person.
Original: In 1998 , Virtual Sheet Music evolved further into what was to be termed Digital Sheet Music , which for the first time allowed for copyright sheet music to be made available for purchase online by the publishers .
Simplified: Sheet music is a subset of printed music . Sheet music usually refers to a `` single sheet '' of music ; that is , one song or piece printed separately .