The origin of a signal is often as important as the signal name.  Therefore, an input symbol can be further constrained by specifying a gate at which the signal must arrive, using a "VIA" clause.  The same signal arriving at another gate will not match the input constraint.

An input symbol constrained by a gate name

Signal parameters can also serve as input constraints.  An input symbol can specify a value for any parameter of a signal.  If a signal contains any other value for that parameter, it will not match.