Refers to a supplementary book or magazine issue published separately from the main series, often with a specific theme or collection.
この全集には別巻として索引が付いている。
This complete works comes with a separate volume containing an index.
雑誌の別巻で特集が組まれた。
A special feature was put together in an extra issue of the magazine.
Compound of 別 (separate) and 巻 (volume). The reading べっかん is a regular on'yomi combination.