There are four basic alarm/limit types, Process, Control Deviation, Rate of Signal Change and Event Based alarm/Limits.*
Process Alarm/Limits are based on the absolute value of the Process Variable. If the PV rises above a Process High limit value, or falls below a Process Low limit value, the alarm/limit will become active.
Deviation Alarm/Limits are based on the value of the Control Deviation error. If the PV is more than the High Deviation limit value above setpoint, or more than the Low Deviation limit value below setpoint, the alarm/limit will become active.
Rate Of Signal Change Alarms are based on the rate of change of the PV. If the rate of change is greater than the alarm value for longer that the Minimum Duration time, the alarm will activate.
Event based alarms/limits activate when the condition for that limit type is true. These can be Signal Break, Low Memory or Loop Control Alarms.