A limiter is essentially an extreme compressor, that brings all audio above the threshold to a predetermined value (the ceiling) no matter how far over the threshold it is.

The Limiter FX unit can be found in the FX section of ATOM:

The unit can be turned on by scrolling down in the fx menu and clicking the top right power icon of the Limiter section. It can also be done through the left-side light in the Limiter module of the fx rack section:

If the red on/off button is activated it means the Limiter is active.

"Threshold," refers to the point at which the Limiter will begin working. Signals above the threshold value will be limited, whereas signals below it will not.

The "Ceiling," sets the max loudness that is allowed to go through the Limiter. Anything higher than the Ceiling value will be limited down to the Ceiling value.

"Release," is the same but for when the compression stops. High release values mean the compression will stop more slowly.

"Mix," is essentially a dry/wet knob for the Limiter. A Mix of 0 means no Limiting is applied, while a Mix of 100 means the entire signal is run through the Limiter.