@openstreetmap/id-tagging-schema/data/fields/source/population.json

The OpenStreetMap tagging model used by the iD editor

{
    "key": "source:population",
    "type": "combo",
    "label": "Population Source",
    "prerequisiteTag": {
        "key": "population"
    },
    "snake_case": false,
    "caseSensitive": true,
    "terms": [
        "census",
        "population reference"
    ]
}