Can I arrange items into unrestricted groups?

I want to be able to define groupings of items without restrictions on how the groupings are related. So a single item may be in many groups, the groups may overlap, or be hierarchical, or be distinct. (I’d even like to be able to put groups into groups.) Is there direct support for that? I can get that effect with tags (by creating a tag corresponding to each group, and saying that an item is in a group if it has the corresponding tag), or by defining a new item type called Group that contains only links to other items. But I don’t want to invent my own mechanism if there’s already a standard way of accomplishing the same thing.

With the plugin CollectionTree, you can create a hierarchy for the collections in your Omeka Classic installation. However, you will still be limited to only putting an item in one collection at a time.

That’s not really what I had in mind. I want to be able to group items in many ways, not just hierarchically. For example, some of the groups I want to define are: 1) photos related to missionary work in China; 2) birthday parties; 3) portraits; 4) house repairs. A single photo may be in several such groups.

Tags would definitely work, as would combining those with exhibits on those four topics. If you’re invested in the concept of “groups” you might also look at Omeka S specifically the functionality of item sets