okay so i have a rough idea of how a deathwatch terminater looks like but what about a marine with scout armour
The same princeable would apply the suit is owned by the deathwatch not the marine his-self so a chapter insigna would have
to interchangable so my question is how would this be done?
It's still typically displayed on the shoulder plate, and I believe is painted on, not engraved. Not that the Deathwatch couldn't have a multitude of spare should plates to be swapped out as need be.
and another thing: I notice that Flak armour is in the rulebook but why space marines where flak armour? in what context would it be a good idea?
Space Marines do not wear Flak armor. It is included in the core rules because Imperial Guard and Chaos Cultists/Renegade Militias wear it, and they are detailed in the Adversaries section. Rather then publish the rules for Flak armor in each description, it is included in the Armory for the sake of brevity.