DMX loss (sometimes referred to as "Hold Last Look") behavior refers to what a DMX-controlled device will do when it loses its DMX input. In newer ETC products, this is a configurable property, where you can choose what the device does when DMX goes away. The device might maintain its levels for a set period of time, forever, or turn off immediately.
In Legacy Unison
Legacy Unison processors (CMEd, CMd, etc) do not have the ability to maintain levels on the loss of DMX. If the incoming DMX signal to a Unison processor is lost, and there are no Presets or Test states active, any previously-active levels will be lost, and lights will turn off.
Please use caution when working with DMX-controlled Unison systems in occupied spaces, to avoid unexpectedly going dark.