= compares whether the two elements on either side the operator are equal. This evaluates to either true or false, delivering different outcomes.
If either side contains an information which is unknown, the whole expression becomes unknown.
p is Personp.domicile = de
Adding this to a document, means that document will only be included in the case when the person involved has residency in Germany. When the person lives in Belgium, the document will not show up in the case.