In this case the input attribute and all its values are made available to the enclosing definition. If an attribute of the same name has already been provided, then the values are combined with it.
Multi-value definitions are <AttributeDefinition> or <DataConnector> types which consume multiple attribute values at once. These values are often, but not always, from one input. Nearly all AttributeDefinition types are multi-value capable. In this case all the attribute values are made available and values from multiple like-named attributes are combined.
The ComputedId and StoredId DataConnectors in contrast require a single value (which may come from multiple sources, all but one of which would need to be empty).