Now go back to the Filter Custom card and select the Flow you just built. The inputs will auto-populate, then you can map data into them. To map an input to accept each item of the list, click in that field and you will see a drop down that includes Item - select that. Or if your list is a list of objects, you may also see the schema of the object, i.e. the keys that it contains, and you can also build your child Flow to accept those as inputs. In addition, you can drag in outputs from previous cards in the Flow - in that case, the same value will be used for each item in this list.
list: The input list that contains all of the items you will test
flow: The child Flow that will be called once for each item in the list
with the following values (dynamically generated): The inputs defined by the child Flow - these will populate after you select the child Flow using the Choose Flow dialog
concurrency: The number of items in the list to process in parallel. If it is important that the items are processed in sequence, use 1. Otherwise a higher number like 5 or 10 will cause your Flow to complete sooner.
- list: The output list that contains only those items that passed the test, as determined by the provided child Flow.