the output returned by the Query Unit can be passed to another unit to be used during a query. For example, a Query Unit has a "Query Output" with the oid of a Product; an Index Unit receives this oid and retrieves the Combinations associated to the Product. In your case:
1) can you explain your current situation and the result you wish to obtain ? For example, you should describe (e.g. with a screenshot) the part of the model with the Query Unit, the properties of the Query Unit,...
2) what is your behavior ? For example, the second query does not return any result, the result page shows an error message,..