Set a condition that evaluates to 'true' if a certain property is not unknown.
Example
p is Person
p.tax_domicile is known
You can have a field on a document for a person's Tax Identification Number (tin). You only want to ask for it, if the country where a person pays taxes is known.
This condition, when attached to the field 'tin', guarantees the tin will only be asked if the tax country is known.
