Os by Petzl : What is the difference between constant and declining mode?
Constant: the main advantage of constant lighting is that it offers stable performance over a specific battery life. This way, the user knows that the brightness and lighting distance will not decrease during this period.
Declining: by comparison, traditional (declining) lighting is very bright at the beginning, weakens greatly about twenty to thirty minutes later, then continues to decrease until the batteries have been drained. This mode offers the best compromise between lighting distance and battery life.