Quotes of interest...
 
"Let us define musical borrowing broadly as taking something from an existing piece of music and using it in a new piece." Peter Burkholder, 'The Uses of Existing Music', Music Library Association Notes, Vol.50 (3),  1994, p863.

"...by referring to other music, all types of borrowing force us to think of another piece of music while we encounter the one in front of us, giving works that use existing music a special place in a musical tradition that esteems both the distinctive contributions of each composer or improviser and the repetition of the familiar.” Peter Burkholder, 'The Uses of Existing Music', Music Library Association Notes, Vol.50 (3), 1994, p859.