The papers published normally in journals are all volumes with page numbers.

Periodical: if a periodical has several issues in a volume every year, and the papers published in several issues have no specific theme and have no influence on each other, it is called periodical;

Special issue: there are two situations, one is the special issue that occupies the page of the main issue,

Special problems other than normal problems.

Special issue occupies the layout of the main issue: it occupies a certain issue of the main issue, with volumes and page numbers, and the published papers are all under a certain theme. For example, a periodical has six issues a year, and the second issue has a theme.