where No is output noise spectral density in dBm/Hz and -174 dBm/Hz is kt noise.

To calculate noise figure we have to give the DAC a gain. While dB gain has no real meaning for a Digital to Analog converter, we can arbitrarily define the gain of the DAC to be 0 dB when the output signal is 0dBFS and the frequency is close to zero. If you change the dBFS level or the frequency (causing Sin(x)/x roll off, the DAC's "gain" will decrease.

And if I use a fraction of the DAC bandwidth, do I gain an improvement in No just like an ADC benefits from oversampling? I.e. if the bandwidth extends only to fs/4 does No improve by 3dB [processing gain 10log (Fs/2BW)]?

DAC noise figure is calculated using the equation

No = -174 dBm/Hz + Gain (dB) + NF (dB)

Therefore

NF = No + 174 - Gain

where No is output noise spectral density in dBm/Hz and -174 dBm/Hz is kt noise.

To calculate noise figure we have to give the DAC a gain. While dB gain has no real meaning for a Digital to Analog converter, we can arbitrarily define the gain of the DAC to be 0 dB when the output signal is 0dBFS and the frequency is close to zero. If you change the dBFS level or the frequency (causing Sin(x)/x roll off, the DAC's "gain" will decrease.