In refining, a constraint refers to some physical limitation on the process such as a capacity limit, quality limit, or operating condition limit (temperature, pressure).
Typically, process engineers are constantly identifying process constraints and looking for ways to overcome them through de-bottlenecking.
In addition to the direct physical constraints in the plant hardware, there are also constraints in the models of the the refinery such as the LP and scheduling model. Often these constraints are attempts to simplify or aggregate the representation of real physical constraints. However, on occasion modeled constraints deviate from reality. These are called false constraints and are essentially errors in the models representation of the plant.