For SQL there is usually an optimization where nullable columns are simply a bit set; same with boolean/bit columns. This does sound like a good use of a nullab

For SQL there is usually an optimization where nullable columns are simply a bit set; same with boolean/bit columns. This does sound like a good use of a nullable bit. You may want to distinguish things further in the future such as adding a new column and you can determine if it was unset/undefined by the user versus null because the user never saw it. In that case an enum makes sense. Personally I think the logic maps directly to nullable bit.