The roster is the entry point for the user to their contacts. It is thus a key part in the user experience of any XMPP clients.
Roster entries are organized in groups. There are two fundamentally different ways to model roster groups in a user interface.
One way is to organize contacts in a tree-like (possibly nested, see XEP-0083) structure, where contacts are nodes which are children to group nodes. If a contact is present in multiple groups, it is is also represented by multiple nodes in the tree. A contact that is not part of any groups is shown at the highest level of the hierarchy or in a "non-group" group with a generic name.
This is the traditional way to do things.
Another way is to treat "groups" as "tags". Contacts are then shown in a flat list and the tags attached to them are shown next to the contact. The filter capabilities of a collapsible tree structure is replaced by allowing to filter for (possibly multiple) tags.