If the data needs to differentiate between those cases and a separate boolean bit of data to track whether it was collected is no feasible (as it so often isn't in the real world), then you do what you must. How that works out in the real world would be the policy for how to interpret the data in that field. It's really no different than any time you've seem a field that is named something counter-intuitive to what it holds in some schema. Something tracks that difference, whether it be institutional knowledge, come conversion code, or a label prior to display. That's what I mean by it being a "schema/data policy question".