Using the Mark and Execute technique, a player can choose to have to have Sam target enemies for quick lethal take downs (generally head and neck shots). This can apply in both stealth and combat situations.
For example, in the E3 2009 gameplay demo Sam is shown is shown spying and marking targets from underneath a door, quickly opening the door and "executing" his maneuver. In this case he was able to quickly take out a light and an enemy silently and efficiently before catching another enemy unaware and grabbing him. All of this happen in under five seconds thanks to mark and execute.
As stated by Creative Director Maxime Beland, mark and execute can be used anywhere. In the same demo mentioned earlier Sam is able to use mark and execute while hanging from a window ledge in order to dispatch two enemies with quiet and lethal headshots. It seems mark and execute can be used for both assault and stealth.
Sam is also able to apply 'mark and execute' to several weapons including a silenced pistol and compact SMG. There is one instance where he marks his targets before kicking down a door and terminating them with a shotgun in seconds. This demonstrates not only Sam's brute force and power but also the relative ease that he can execute this.