Deus Ex: Human Revolution, like its forebears, allows players to vary their approach, from direct combat to stealth, based on personal expression rather than the requirements of a specific set-piece. The mechanics and level design combine to allow the player to engage in any encounter (excluding boss fights) through the use of stealth, combat, or some combination of the two. Key to this is the cover system around which stealth and combat are built.
In order to allow stealth-focused players to traverse an area successfully, levels need to be designed to allow those players to move between important locations while remaining concealed. In an occlusion-based stealth model this means providing occluding geometry, cover, in a pattern that allows players to move from one location to another while keeping the object between themselves and any NPCs. The combat-focused player is in a similar position, though they will not need the complete path through any encounter space; the more cover available the greater their range of tactical movement options. The same distribution of cover that provides concealed movement can be used by the combat focused player to reposition and potentially outflank the hostile NPCs.
This spatial arrangement of geometry is enhanced by the switch from first- to third-person once players take cover. If the game remained in first-person when Adam Jensen took cover behind a lab bench or crate it would be difficult to maintain situational awareness. For both the stealth and combat focused players an accurate mental model of where NPCs are within a given area is important; for the former it’s necessary to ensure that you keep some form of solid object between you, for the latter you need to know where somebody is before you can shoot them.
Deus Ex: Human Revolution does provide a radar display that can serve as a guide to the relative position of hostile NPCs in the environment, however it operates at a level of abstraction and therefore using it becomes a two stage process. Instead of looking at the world and seeing exactly where NPCs are positioned players relying exclusively on the radar will need to mentally overlay the information provided onto what they remember about the spatial layout of the level. Where the radar excels is in its ability to provide information on NPCs outside your immediate field-of-view, helping you to avoid being flanked.
Deus Ex: Human Revolution uses its cover system for both combat and stealth and in doing so has managed to enable the two to coexist in a way that allows both to be utilised within a single encounter. The same occluding geometry that provides concealment from detection also provides protection from incoming fire. Instead of making the stealth-focused player weak by forcing them into the darkness and away from enemies, the cover system of Deus Ex: Human Revolution ensures that stealth-focused players are in just as strong a position as combat-focused players when in cover.
By requiring both stealth- and combat-focused players to relate to the spatial layout of a level in the same way Deus Ex: Human Revolution is able to create a hybrid system where mastering the core abilities of movement and positioning are beneficial to all types of player. Furthermore, when the same layout of cover is beneficial to multiple approaches it makes it easier to switch between styles, even within the same encounter.