A set coil separates the coil from the action or condition that energized the coil. Once energized, a set coil's result is no longer dependant on the action that energized it. A set coil stays energized (latched) until its condition is reset (unlatched) by a reset coil. You can link it to any of the following bit operands:
An example of a set coil is an overhead light. When you turn on a light, it stays lit until you turn it off (reset or unlatch it) or the light bulb burns out. You do not have to hold the light switch to keep the light on.
An example of a coil that you do not want to be set ( latched) is a car horn. You expect it to toot only when you press on the horn button and you expect it to stop when you stop pressing on the horn button.
Do not use a set coil without a reset coil in a program.