The low impedance means that reasonably good isolation between two loads may be achieved simply by connecting the loads to the oscillator though 50 ohm resistors. The low-Z output is also more efficient since no power is dissipated in an internal resistor. A 10 dB return loss means that the reflected signal will be 10 dB below the incident signal. A common specification is 14 dB which is a VSWR of 1.5. See the loss chart .
Return loss is a popular way to specify the output impedance since it is easily measured.
A signal near the oscillator's frequency is applied to the oscillator through a directional coupler or isolator.