Dwell volume or system volume specification for an HPLC or UPLC system is the volume from where the gradient is formed to the inlet of the column. However, for the user the gradient delay is the volume or time required for the gradient to get through the system and the column(s) to the detector or mass spectrometer. In the case of nanoACQUITY, the dwell volume is <1uL. I estimate the total volume of system volume, trap and analytical columns (Vo), connecting tubing, and Universal sprayer to be ~1.8uL.
Waters cites that the volume of the nanoACQUITY UPLC system is < 1 uL from point of gradient mixing to the head of the column, loop off-line. (i.e. trap column volume is not included).
I believe that Jeanne's answer is as comprehensive as it gets!