Just so I don't appear to be too RTFM-y, here's an example of RNGMERGE usage for a SmartSheet that can be expanded or collapsed, using the Departmental Comparative process as mentioned above:

=GLAMT(H$8,RNGMERGE($B11,STRCAT("-",H$6)))

Where cell H8 contains the field code to return, such as "M" for current period net changes or "Y" for YTD net changes
Cell B11 contains the Account number (no departments or dashes)
Cell H6 contains the selection of departments, such as 10,82 without any dashes.