noun
separate-volume supplement
A booklet or insert that accompanies a main publication, such as a magazine or book, and is bound separately.
この雑誌には別冊付録がついています。
This magazine comes with a separate-volume supplement.
付録 is a general supplement or appendix, while 別冊付録 specifically refers to a supplement bound separately from the main volume.
Compound of 別冊 (separate volume) and 付録 (supplement).