Compound sensors are very similar to absolute sensors in that they both measure a vacuum. The difference is how the vacuum is displayed by the sensor.
In a compound pressure sensor, the sensor is zero referenced to atmospheric pressure. When measuring a vacuum, a compound pressure sensor will display a negative number. For example, a 10 PSI vacuum would display as -10 PSI with a compound pressure sensor.
In an absolute pressure sensor, the sensor is zero referenced to a full vacuum. When measuring a vacuum, an absolute pressure sensor will display a positive number. Using the same example as the compound pressure sensor above, a 10 PSI vacuum would display as +10 PSI with an absolute pressure sensor.
0 PSI absolute is equal to about -14.7 PSIG.