Rev Acting
This stands for reverse acting in some circumstances the relay that is connected to the output actually works BACKWARDS!! (normally closed contacts)
This would normally make things very confusing as when things should be on they are actually off and when things should be off they are actually on ie. everything is working backwards.
The selectable rev act feature to combat this problem. When Rev Act is OFF the o/p will turn on when it should be on and off when it should be off (normal Logic).
When Rev Act is ON the o/p will turn off when it should be on and on when it should be off (reverse Logic). Rev Act would normally be OFF only turn it ON if you are using a backwards relay. (with normally closed contacts)