The two devices that you want/wish to be given priority with respect to network communications between the two.
Source being where the data originates. Destination being where the data is going.
For example: if you have four computers/devices - A, B, C, and D
And you want A's and D's network communications to be given priority over all other network communications then you would use the IP addresses for A and D.
You could for example, set up rules to give your computer priority with respect to traffic between it and the router. Appears that you need to do so for upload (data from your computer) and download (data to your computer).
Plus those device IP addresses would need to be static (not changing). If either IP address...