About selecting one item

Use the Select One Item flow action to check conditions to extract a specific item from a set of items.
The item that meets the condition goes into the Item result. RBA ignores items that don't meet the condition. Actions that follow in the rule set are performed only on the selected item.
The most common use of Select One Item is when you use a page set event, such as Page Set Full, with the Create Imposition action. Since a job can have multiple page sets, use Select One Item to identify the specific page set that will be used for the imposition.
When you need to process all the items in a group, one at a time, use the Split flow action.