Experience

Experience Points (XP) are an abstract measure of the experience, knowledge, and training that a PC gains while adventuring. They are gained from RPBonus, beating monsters, or completing rare quests.

The scripting formula used for monster XP awards is quite complicated and its exact definition on Arelith is not a public knowledge. Some factors taken into consideration include:

Challenge rating of monster
Level of highest character within sight of a creature
Number of characters within sight of a creature ("Party size")
Number and level of summoned familiars, summons, animal companions and other friendly monsters within sight of killed monster
Level of your character
Effective character level (ECL)
~~Multiclassing penalty~~ Multiclass Penalty has been removed from Arelith.

Description by Mithreas:

Base XP for all characters is calculated according to the highest level character present
It's then modified for each person independently according to ECL (if any) and/or the bonus we give to chars under level 7
Finally, we apply a party size modifier. A party of 4 gets the best XP, and XP tails off rapidly as your party gets bigger than 6 people. Only PCs are counted for the party size count - a mage with a familiar and two undead summons still counts as a 1 person party for that part of the calculation. NPCs present will reduce XP via a complex formula based on hit dice.

ECL doesn't reduce the experience gain from RPBonus bonus.

External links

Experience point on NWNWiki