RAM disk is the area of internal RAM where files are stored. System RAM is the area of internal RAM where programs are executed. The default (an even split) is best until you have a reason to change.
You should expand your RAM disk if you have no more space to store files.
You should expand your System RAM if you don't have enough space to run your programs or your Lotus spreadsheet has the MEM indicator showing. You can also free up system RAM for your Lotus spreadsheet by closing any Phone, Appointment, Comm, Calculator, or Memo applications which are open.