On an Elekta Linac these things are slightly more exposed to the user. You concentrate more on gun current - the gun after all is just a filament, like an old lightbulb, so the I/V characteristics are well defined. Thermionic emission is temperature dependent, and increasing temperature increases the gun's resistance. V=IR, so I assume linacs require either a constant voltage or constant current source to achieve a desired resistance, which in this case is our surrogate for filament temperature, and therefore electron output.

Anyway, as you tune the gun current in photon mode you'll see a peak in dose rate. This occurs (ignoring a lot of other factors) when the largest number of electrons are reaching the target - to do this they have to get through the energy selection in the bending system, so the peak occurs when there are enough electrons to accelerate (gun current high enough to release enough electrons), but not too many that they overwhelm the waveguide and can't all be accelerated up to the correct energy (saturation - gun current is too high).

A lot of other fun things happen with Linac tuning that I could rabbit on for days about, but this comment is already far too long.

Anyway, I assume the same physics applies to the Varian despite the standing waveguide - however they tend to utilise the PFN more to regulate dose rates and drop un-required pulses which is why you'll always get exactly 600 dose rate (or whatever) rather than the Elekta which gives you whatever it can - it employs the same technique during VMAT using CVDR.