In Aion 2 Taiwan, item level is calculated from equipment and 
Daevanion; combat power is an indicator of PvE strength calculated from character stats. Consumable and skill buffs are excluded from combat power. A higher item-level piece therefore does not establish a higher damage result for the activity you are playing.
What raises item level?
Equipment grade and enhancement contribute to item level, along with Daevanion. Check the item-level figure when an activity names it as an entry requirement. Your character level is a different requirement: a weapon's item level and the level needed to equip it are separate fields.
For example, the 
Noble Dragon Lord Greatsword is item level 108 and the Splendent Noble Dragon Lord Greatsword is item level 114. Both require character level 45. The larger item number does not change that equip requirement.
What does combat power count?
Combat power weights character attributes according to their PvE importance. Different stats can therefore move the total by different amounts. Temporary gains from consumables or skills do not count toward it.
That makes the number useful for checking your character's PvE stat development, but it does not measure a particular attack sequence, mechanic execution or a PvP matchup. Check the attribute you need rather than treating every point added to the total as equal damage.
The display can switch between two measures with different inputs.
The character's item-level display can switch between item level and combat power. Check which figure a party or activity is asking for before comparing your number with it.
How do you judge the item instead of the score?
Compare the same equipment category and read its individual attributes. The two greatswords above have base Attack ranges of 463–627 and 489–661 respectively. That is a concrete +26 / +34 change. The worked equipment comparison also checks their random-attribute slots, faction, trade rules and set relationships.
PvE and PvP damage attributes are different lines. A weapon that helps one mode is not automatically a better choice for the other because its item level is higher. Likewise, a possible random roll is not an attribute already present on the copy you own.
The two swords differ by 26 at the low end and 34 at the high end of base Attack.
What should you check before joining a group?
Read the activity's required character level and item level first. A party's additional combat-power request is a separate recruitment condition, not evidence of the game's exact damage formula. Check your relevant stats and skill choices after meeting entry requirements. The class pages and party-effect guide help distinguish your role from the number beside your character.