Relative is relative to the fundamental frequency. Absolute is relative to 0. So, if you sweep from 1Hz to 1MHz on a 1GHz oscillator and sweep type is absolute, then the noise will be measure from 1Hz to 1MHz. However, if the sweep time is relative, then it is measured from 1GHz + 1Hz to 1GHz + 1MHz.

The reason you use relative for oscillators is that you almost always want to know the noise near the carrier frequency, and the carrier frequency is not known precisely. In my example I said the oscillation frequency is 1GHz, but it is only approximately 1GHz. So you need a relative sweep, an absolute sweep is useless for oscillators.

With driven circuit, you do know the fundamental frequency precisely, so the need for relative sweeps is reduced. However, you are free to use either sweep type on driven circuits as suits your needs.

-Ken