別冊

1. Origins, structure, and basic meaning

• Structure: 別冊 (べっさつ) is a supplement or volume issued separately from a main publication. It appears as an attachment and a separately bought item.
• Basic usage: A magazine supplement carries a detailed map; a separate volume of feature articles is purchased.

2. Common pitfalls and similar expressions

• Supplement tied to a magazine and separate volume sold
  • 雑誌の別冊 highlights relation to the main issue; 別冊を購入 treats it as merchandise. It need not be free.