Complex sentence: It is also notable for being the first game along with "Half-Life 2" to use Valve's Source engine, which allows the game to be played from either the first-person or third-person shooter perspective.
Split into simpler sentences: It is also notable for being the first game along with "Half-Life 2" to use Valve's Source engine. The game can be played from either the first-person or third-person shooter perspective.