Some journals have volumes and issues or parts. This structure is a legacy from print journals, where a collection of articles would be published together in an issue or part. An issue would be printed at intervals throughout the year, perhaps every month or every quarter. In the library, issues were arranged in order on the shelves. At the end of each year, they would be stitched together and given a hardback cover for protection, and this collection of issues was called a volume. The volume may have been named after the year (for example, 1997), or given a number (for example, 40).
In this way, each journal was made up of volumes, each containing a number of issues, and each containing a number of articles. Although many articles are now published electronically, rather than in print, the same structure is often used for online journals.
Learn more about DOIs at different levels, and journal-title-level, journal-volume-level, and journal-issue-level DOIs.