Voltage controlled oscillators such as Voltage Controlled Crystal Oscillator(VCXO) are typically coupled via a phase locked loop( PLL) and must be able to be varied slightly in their frequency range.
These slight frequency variations are required when an output frequency changes due to temperature influences. The voltage-controlled oscillator is then adjusted via the control loop.
The Absolute Pull Range (APR) is defined as the controllable frequency range of a VCXO with all tolerances taken into account. The Absolute Pull Range is a result of the control characteristic, the frequency stability of the VCXO, and its aging characteristics. The APR values are in the ppm range and say by how many parts per million the oscillator frequency can be "pulled".