“Completed” field in game is for when you’ve finished a game.
That’s why that is not a field in Hardware.
“Completeness” however, on the value tab, is a field for games and hardware. You can enter if the hardware item is “New”, “Complete in Box” or “Loose”. Depending on what you choose, the value also changes.
As for adding your own fields: that is a feature that is coming up for early 2026!