An open system can exchange both matter and energy with the surroundings. Δ H is measured when
an open system is used as a calorimeter.
A closed system has a fixed amount of matter, but it can exchange energy with the surroundings. Δ U is
measured when a closed system is used as a calorimeter because there is no change in volume and thus no
expansion work can be done.
An isolated system has no contact with its surroundings. The universe is considered an isolated system but on
a less profound scale, your thermos for keeping liquids hot approximates an isolated system.