Why do data sources matter?
A source shows where a field value came from. It lets editors check a claim, compare conflicting evidence, and correct errors.
When you change a field, add the source that supports the value. A source can be a document, a website, a measurement, or an observation. A value without a source is harder for another editor to verify.
Select an existing data source if it describes the evidence you used. Add a new source if none fits. Include the exact page, URL, document title, or observation circumstances when you have them. Date witnessed is the date on which you accessed or observed the source. It is not the date on which the building fact became true.
Sources help when two records disagree. Editors can inspect the underlying evidence instead of relying on the value alone. If you do not know the source, do not invent one. You can still enter the value and add evidence later.