MNI

MNI (Multiple Node Instances) is configured on the Other tab of your process model nodes.  This allows you to run multiple instances of the node, either at the same time, or one at a time.  Simply select "Automatically run multiple instances of this node", then choose to run a set number of instances or "Run one instance for each item in [process variable]", the latter being my most common use case.  Documentation is here.

I've been having to utilize these less and less with later versions of Appian, but there are still use cases - mostly in nodes that only allow a single input but you want to process a list.  In one process we run the Delete Document node as MNI for daily cleanup on reports generated/saved for export (as this node can only be run on a single document at once).  In another we use MNI to deactivate a list of users.  Another posts system events to the Tempo News Feed as MNI.  In another process we utilize a sub process as MNI where a list of users are each sent a task for commenting, where other processing occurs around each task.  

Probably the most helpful un-documented feature of MNI is tp!instanceindex, which is used to determine the current index of the MNI node.  Say you are running a script task over a multiple valued pv!myVariable, inside the script task config pv!myVariable[tp!instanceindex] will get you the corresponding value to the node index.

As there are some limitations to MNI and potential server cost for node executions/process history, I would only recommend MNI when you have a use case that cannot be achieved in another fashion such as a!forEach() in a SAIL expression, rather than looking for reasons to implement MNI for the sake of MNI.