A Scan measures when your URL link (PhyID) is scanned or tapped.
An Impression measures when an Action on your URL is displayed.
A Tap measures when an Action is clicked.
Impressions become very important for users who update their content often. If an Action was added halfway through a reporting period, it will have have the impression as one there the whole time. If both have the same number of taps, they the one there half the time performed better. Tracking impressions allows you to see these performance differences without guessing.