A different pressure click. The "Dif" stands for differential
Diff click has two ports for industry standard pressure tubing. For rough testing you could also use two straws. Like a cocktail for two, a friend and you could blow through two straws to determine who has more lung capacity.
The upper port is P1 and the lower is P2. P2 should be the referent pressure gauge, labeled vacuum in the data sheet. So P1 should be larger than P2, otherwise you would get a negative reading.
These kinds of devices are useful for monitoring machinery. The data sheet lists measuring water levels in washing machine as an example.
In case the second tubing port is redundant for you, don't forget that we have Manometer click with a single port.