Confusion with the meaning of current regulation for this context
In this circuit, the current is fed to the crystal and the charge amplifier to supply them, but at the same time, it allows the signal sent back to oscillate.

If you didn't had the current limiting diode, the signal would be fixed at VCC and you wouldn't be able to measure anything.


  My confusion here is that what is meant by current regulation here? Is it keeping the current constant all the time at a particular value or limiting the current?


Both are quite the same thing, it is limiting the current provided from the DC supply and also keeping the current constant all the time, by limiting it.


  Yet the value of the decoupling capacitor Co below seems up to the user.


Yes, as it is used as decoupling and also high-pass filter. Co will have an effect, depending how big is Co, it will filter lower frequencies oscillations.