Sorry, but in the PDF that is out now there is no mentioning what Stunned means. Therefore I think that it will also not be mentioned in the book that I get today (Yeah!!!). That is a little bit sad. And is it really that you lost all actions when stunned?
If unsuccessful they are stunned and lose their next slow action.
What is correct? As a Flash Grenade doesn't do Stun but you lose your next Slow Action I think correct is you lose all actions.
I was extrapolating from Coriolis which is a slightly different ruleset but has crossover with the Alien RPG. Having had a look at the Flash Grenade on page 125 I think (opinion only) that is description of the Stunned Effect. When something has STUN listed as the damage type, you have to roll a successful STAMINA check or lose your next Slow action. The Flash grenade doesn't have its own damage entry as it's rolled in with the U1 Grenade Launcher.
However ideally the STUNNED condition should probably have its own box on Page 106 "Other Hazards" or STUN damage should have an entry on Page 98 "Damage" in my humble opinion. Unless of course STUN damage and the condition of being STUNNED are two different things.