question

First you can keep all items in the rack by the picklist option " Do Not Release Items" in the Flow tab Send To function. Now it is on you to release the items individually. The command that do that is "releaseitem(obj item, output port number). You need to identify the items in the rack you are going to release if your condition is fulfilled. Classically you are counting the different types in labels by incrementing them when an item enters like @Daniel C suggested. Further you can put the reference to the items on a list or into table. You release the items by the above mentioned command in a loop and use the references from the table or the list by pulling the list entries or you look for the right references in the rack directly by a query over the object. The last approach is well documented in the manual SQL section. Please come back if you get into trouble realizing you approach and ask new questions.