Collections differ in so much as they are not a primary issue for a paper but tend to be collections of papers with similar topics across multiple issues. So an article may be in the Thomas Pynchon Collection but its primary issue may be Volume 1 Issue 2 2019. You can also define your own issue types in the Django admin area.

Volume 0 can be helpful for collections, if you do not wish for the collections to follow or interrupt the regular Volume-Issue order. (E.g. for collections you may continuously add to and arent part of specific years.)