What's the difference between sensitivity and resolution of an SMU?
Resolution in this context means the resolution of the display, which depends on the number of digits:


  A typical digital electrometer might have 5 1⁄2 digits, meaning five
  whole digits (each with possible values between 0 and 9) plus a
  leading half digit that can take on the values 0 or ±1. Thus, a 5
  1⁄2-digit display can show 0  to 199,999, a total of 200,000 counts.
  The resolution of the display is the ratio of the smallest count to the maximum count (1/200,000 or 0.0005% for a  5 1⁄2-digit display). (same source)


Regarding sensitivity,


  The ultimate sensitivity of a measuring instrument depends on both its
  resolution and the lowest measurement range. For example, the
  sensitivity  of a 5 1⁄2-digit DMM with a 200mV measurement range is
  1μV. (same source)