This is fairly straightforward, but to be specific, you're goint to have to provide more data on your setup.
In general, if you are trying to populate a single datapoint (like a text input or label) from a datasource based on the information in another item on your screen, you will want either:
LookUp(yourSource, [your condition statement]).yourField
--or--
First(Filter(yourSource, [your condition statement])).yourField
If you are trying populate a list (or gallery) of items based on the information in another item on your screen, you will want:
Filter(yourSource, [your condition statement])
In any of those instances, it is the [your condition statement] where you would create the linkage between the two fields. For instance:
yourNAVPackingSlipNumber = yourSCANPackingSlipNumber
I apologize if that is all already known to you and you're actually looking for help on a different part, but you left so much unsaid about your setup that I figured I'd better start at the beginning.