We don't put enforced constraints on the user columns to make deleting users easier and to preserve the user id value even after the user is deleted. Most of the tables in LabKey have a CreatedBy and ModifiedBy column that don't have an FK to the users table. As an aside: the Owner, Created, CreatedBy, Modified, and ModifiedBy columns will be populated for you when you insert or update a row.

The xml schema reference documents the various metadata available in the schema.xml file:

https://www.labkey.org/download/schema-docs/xml-schemas/schemas/tableInfo_xsd/schema-summary.html

The same metadata is used for custom queries as well:

https://www.labkey.org/wiki/home/Documentation/page.view?name=metadataSql
https://www.labkey.org/wiki/home/Documentation/page.view?name=columnMetadata