Which CONTROL PREAMPLIFIER control the VOLUME?

Say, one has a regular preamplifier with a volume control knob, and then one adds a DAC to the stereo system, which has its own volume control knob.
 WHICH VOLUME CONTROL KNOB actually DETERMINES the VOLUME LEVEL- the one on the preamplifier or the one on the DAC?! Or do they just battle it out between themselves,eh?!