Normally when joining a MUC groupchat, it is each individual client that joins. This means their presence in the group is tied to the session, which can be short-lived or unstable, especially in the case of mobile clients.
This has a few problems. For one, for every message to the groupchat, a copy is sent to each joined client. This means that at the account level, each message would pass by once for each client that is joined, making it difficult to archive these messages in the users personal archive.
A potentially better approach would be that the user account itself is the entity that joins the groupchat. Since the account is an entity that lives in the server itself, and the server tends to be online on a good connection most of the time, this may improve the experience and simplify some problems.
This is one of the essential changes in the MIX architecture, which is being designed to replace MUC.
mod_minimix is an experiment meant to determine if things can be improved without replacing the entire MUC standard. It works by pretending to each client that nothing is different and that they are joining MUCs directly, but behind the scenes, it arranges it such that only the account itself joins each groupchat. Which sessions have joined which groups are kept track of. Groupchat messages are then forked to those sessions, similar to how normal chat messages work.
Briefly tested with Prosody trunk (as of this writing).