The realism is introduced through a number of changes. The weapons available are real-life, have recoil, are less accurate when fired while moving, have limited ammunition and the need for a lengthy reload after a clip has been expended. The number of weapons and other gear that can be carried is limited. Damage is also realistic, based on dividing the player target into discreet areas, hits are frequently deadly - a single head-shot will kill anyone; without protection chest hits will kill instantly; leg wounds will slow you down; and not stopping to bandage wounds will lead to death from bleeding. Lots of sprinting, jumping and climbing reduces stamina. Depending on the map used external environments are more realistic too and can include weather effects such as rain or snow.
Game modes are similar the original game (FFA, TDM, CTF) but have additional team survivor, follow-the-leader, captue and hold, and bomb-defuse games.