A "feature upgrade" involves enabling additional constellation capability within the GSS6300 or GSS6700 simulator. For example you may initially start with a GPS only unit and later on decide to purchase GLONASS and/or Galileo capability.
If you are interested in upgrading the capability of your GSS6300 or GSS6700 simulator please contact your local sales channel for further information.
Once purchased you will receive a new feature key and, for GSS6700 users when applicable, a new sig_gen.txt