Difference between special issues and regular ones
Different journal may put different meaning into "special issue" It could mean that papers have been invited around a certain theme, it could mean papers come from a workshop or symposia, or that the journal have decided to gather papers around a specific topic for some reason. The point is that a special issue differs from a regular issue in that the papers constitute a collection around a theme or coming from a specific group of authors or event. Such issues can either be a decision by the journal or a request from a group of scientists to publish around "their" proposed theme. There may be additional cases but these are the ones I am familiar with within my field.