When constraints are evaluated, the SQL that is sent to the database in most cases is not the literal constraint SQL Statement. The Rules Engine attempts to add WHERE conditions to the SQL statements in which column values for only one record are substituted. This is called row binding.
For each constraint there is only one table for which row binding applies. This is called the driving table of the constraint. When a constraint is parsed, the Rules Engine identifies its driving table. This is the table "from which" the constraint is evaluated.
"Driving table" is a term that some RDBMS vendors use in a different way.