Requirement

General
Main items as well as off-hand items pose an attribute requirement in order to use them to their full benefit. It is a common misunderstanding that items for which a player does not meet the requirement do not give any benefit at all - the gain is mitigated somewhat, but not obliterated completely:
 * For melee weapons, bows and spears (see the different skill types), the min. and max. damage is reduced by 1 for every point the requirement is not met. Example: A Warrior wielding a max dmg Axe (6-28) with req. 9 in Axe Mastery will only do 4-26 damage when having his Axe Mastery set to 7.
 * Off-hand focus items and main-hand caster weapons - e.g a holy rod - that provide a energy bonus have their gain mitigated based on the value of their requirement. The following table consolidates the observations from several testers, which were also confirmed by others:

Preliminary Conclusion

 * Speaking in general terms, there seems to be a "mitigated gain" cap (that is, the absolute maximum energy gain when not meeting the requirement) of a total of 6 energy.
 * The most common req. 9 items (as they include the collector items) seem to have the biggest drawback with respect to the mitigated energy gain, which is only 25%.
 * Be aware that Weakness causes all attributes to drop by -1, so meeting the req. demand by even +1 should be the wisest choice, especially when equipped with one of the wide-spread req. 9 items.