The phrase "optional upgrade" means that you have the choice to get a better version or add-on to something, but it is not necessary and you can still use or have the basic version without it. Full definition
The budget - friendly options come with capable hardware with optional upgrades for gamers who want something extra. (techtimes.com)
Some policies have optional upgrades for cancel for work reasons and a few companies offer specialized products for the adventure traveler. (squaremouth.com)
Optional upgrades included leather seats, a sunroof, and a CD changer. (automotive.com)