Alessandro, I pursued this same question yesterday and found an answer.
In an informative BGG thread from 2010 (
boardgamegeek.com/thread/506217/i-need-t...ion-of-when-cards-sa
), we learn the following:
** Cards that read "As an action . . ." means "As long as you do not exceed the number of actions in the action turn you have, you may - as an action - dispose of the card´s positive effect. Bring to bear all three uses in one turn if you wish." This includes the Rally Cry, Desperation, and Dust Cloud cards.
** Cards that read "Once per action turn . . ." means "The only difference is that you may not use the effects of the card (three in total) in one turn. Here you may not, as you say, burn all its uses in one shot." This includes the Take Cover, Critical Objective, and Increased Visibility cards.
Note for the second bullet above the thread also clarifies that for the Take Cover card, this includes Normal, Suppressive, _and_ Op Fire attacks, which means a squad taking the Advance or Fire & Movement actions can use this card against an Op Fire attack.