Need detailed working or explanation of SCAN register in DSPIC
From dsPIC30F2010 datasheet,


  The inputs are selected by the ADCSSL register. If a particular bit in
  the  ADCSSL register is ‘1’, the corresponding input is selected. The
  inputs are always scanned from lower to higher numbered inputs,
  starting after each interrupt. If the number of inputs selected is
  greater than the number of samples taken per interrupt, the higher
  numbered inputs are unused.


Code example is here (for 4013, but the idea is the same)