By default Xcas uses and displays 12 significant digits, but you can set the number of digits to other positive integers. If you set the number of significant digits to a number less than 14, then Xcas will use the computer’s floating point hardware, and so calculations will be done to more significant digits than you asked for, but only the number of digits that you asked for will be displayed. If you set the number of significant digits to 14 or higher, then both the computations and the display will use that number of digits.
You can set the number of significant digits for Xcas by using the CAS configuration screen (see subsection 3.5.7). The number of significant digits is stored in the variable DIGITS or Digits, so you can also set it by giving the variable DIGITS a new value, as in DIGITS:= 20. The value will be stored in the configuration file (see subsection 3.5.10), and so can also be set there.