The scope of a single-record constraint is restricted to the columns of one table. In the constraint you either use a SELECT or UPDATE statement.
Consider the business rule: "A reservation can only be canceled after the reservation is booked." This rule is implemented by a restrictive single-record constraint:
SELECT '' violation
WHERE cancel_date < book_date
The Rules Engine validates single-record constraints at record validation time, when the record is stored. Corrective single-record constraints are validated before restrictive single-record constraints.