Certain subraces are considered to be more powerful than other races, typically because they feature ability bonuses with no penalties. These races are given a level adjustment that modifies the amount of experience the character must gain in order to increase in character level.
A character with a level adjustment requires more XP to reach level 2. Once the character reaches level 2, he levels up at the same rate as other characters, but he will always be one or two levels behind characters that do not have a level adjustment.
A character with a level adjustment can reach the level cap of 20 provided the campaign or module supplies enough XP. In the official campaign, a character with a +1 level adjustment will usually have this happen during the large battle at the keep. With a character with a +2 level adjustment this usually happens close to the final battle. However, characters with an adjustment of +3 will most likely not be able to obtain the level cap of 20 by the end of the official campaign unless they employ "milking" strategies to obtain additional experience points from re-spawning monster areas.
It should be noted that in NWN2, ECL only affects the experience needed a given level. It does not affect the 'effective level' or 'challenge rating' of a PC. This means that a level 5 drow fighting solo will earn the same XP for killing a dragon as a solo level 5 human would.
3.5 Rules ComparisonEdit
Subraces that can be used as player characters have a level adjustment, which is a number that is added to the creature's total hit dice to arrive at its effective character level (ECL). A creature with multiple special abilities is more powerful as a player character than its hit dice alone would indicate. For example, a drow elf has spell resistance, bonuses to its ability scores, and spell-like abilities. Its level adjustment of +2 indicates that a 1st-level drow wizard is the equivalent of a 3rd-level character.
Level adjustment is not the same thing as an adjustment to a creature's challenge rating because of some special qualities it possesses. Challenge rating reflects how difficult an opponent is to fight in a limited number of encounters. Level adjustment shows how powerful a creature is as a player character. For instance, a drow receives a +1 adjustment to its challenge rating to account for its special abilities, indicating that it's tougher in a fight than its hit dice would suggest, but its level adjustment is +2 to balance its abilities over long-term play.