Item Level
Item Level (IL or iLvl) of gear determines that gear's strength. Gear with a higher IL will essentially always be better than gear with a lower IL. This is especially significant at level caps, including former ones, since while gear could no longer increase in level, it could still grow in stats as reflected by its higher Item Level.
Player characters have an average item level that is the sum of the IL of all the equipped pieces of gear divided by the number of slots. Unless the player is equipped with a Shield, the main hand weapon's IL is counted twice in place of the empty off-hand. Thus, when trying to reach a required minimum item level for a duty, it can be best to upgrade the weapon first, even for 
tank or 
healer classes.
A notable caveat is with craftable gear. Only High Quality gear has appropriate stats for its Item Level. Any (NQ) Normal Quality gear has a 10% penalty in stats versus same-IL Aetherial, Green, or Blue rarity gear. This is only a critical issue starting at level 50, once the stats on gear get larger than single digits. NPC vendors only ever sell (NQ) Normal Quality gear; the sole source of High Quality gear for 
Gil is from other players via the Market Board. Leveling gear can also be acquired High Quality from 
Side Quests, some of which award coffers.
Two pieces of gear with the same item level (assuming HQ items if applicable) equipable by the same jobs provide the same increases in vitality and main attributes (i.e., strength, dexterity, intelligence, mind) but may differ in the number of materia melding slots and secondary attributes. The "stronger" item is therefore the one that maximizes the most desirable substats (usually Critical Hit) when fully melded.
Item Level vs. Class Level
From levels 1-49, there is a close to 1:1 conversion between the level requirement to equip a piece of gear, and the item level of that gear.
At level 50 and at later expansions' level caps (60, 70, etc, commonly called x0), there is a larger range of item levels - this is due to higher-item-level gear being gradually released over the course of several major patches (~2 years worth of patches) when the relevant expansions were the current endgame.
In the "leveling" ranges of each expansion (51-59, 61-69, etc, commonly called x1-x9), item level usually increases by 30 before reaching level cap gear.
At a level cap, there are three "tiers" of item levels, spanning 100 item levels (a consistent pattern since Stormblood).
- The first tier spans 40 item levels and is fully released with the x.05 patch of an expansion. During the expansion launch (x.0), only the first 20 item levels are available.
- The second tier spans 30 item levels and is released in the x.2 patch.
- The third tier spans 30 item levels and is released in the x.4 patch.
Each increase in maximum item level past the initial release of an expansion corresponds with the release of a new Allagan Tomestone and 8-player 
raid tier.
| Class Level Equippable | Max Item Level |
|---|---|
| 50 | 130 (135 weapon) |
| 60 | 270 (275* weapon) |
| 70 | 400 (405 weapon) |
| 80 | 530 (535 weapon) |
| 90 | 660 (665 weapon) |
| 100 | 760 (765 weapon) |
* The Lv. 60 Coven Weapons are IL 280 but are no longer obtainable.
These maximum item levels are used on any former endgame content that lacks a more-specific Item Level Sync. For instance, 
Snowcloak only syncs to level 50, so the effective maximum Item Level will be 130.
Leveling (non-level-cap) content without specific Item Level Syncs, will sync to the max IL achievable by gear matching the class level of the duty. For example, Doma Castle is a level 67 dungeon, but players are synced down to level 68 if they are level 69 or higher. Since the dungeon doesn't have a specific Item Level Sync, it syncs to the highest IL achievable by level 68 gear.
Maximum item level is expected to increase to 790 (795 weapon) in patch 7.4 until the next expansion (patch 8.0).
Item Level Sync
When the player's class level is synced down, the item levels of the player's gear are capped according to the target class level. For example, at class level 50, the item level cap is 130. Some duties will have an additional item level sync, which applies a lower item level cap than usual - these lower sync caps primarily appear in duties at the current or previous class level caps (50, 60, 70, etc).
A piece of gear that is synced down will have its materia melds ignored, and its attributes are capped according to the target item level. For secondary attributes (i.e., substats such as Critical Hit and Determination), the cap is equal to the higher substat of a high quality crafted gear, dungeon, tomestone, or trial/raid gear of the same type (weapon, head/arms/feet, body/legs, or accessories) at the same item level.[1]
For example, item level 475 accessories have 88 as the high substat, so with an item level sync of 475, any accessories with a higher item level will have both substats capped at 88. Substats will not be scaled up.
| IL 500 | IL 600 | |
|---|---|---|
| Substat A (base → synced) | 93 → 88 | 127 → 88 |
| Substat B (base → synced) | 65 → 65 | 89 → 88 |
In this example, syncing down any accessories with at least item level 600 will cap both substats.
Substat caps vary by the type of the gear. For example, the following are the substat caps for IL 475 gear:
| Weapon | Head/Hands/Feet | Body/Legs | Accessories |
|---|---|---|---|
| 184 (  PLD: 131 sword + 53 shield) |
112 | 177 | 88 |
- For gear that occupies multiple slots, the cap is the sum of the caps across those slots.
Note that syncing down sufficiently high item level gear to cap substats may be very slightly worse than using a melded item matching the item level sync, but this depends on the specific gear pieces, the content, and job.
However, for the most recent ultimate raids, players are expected to use gear matching the item level sync for the encounter. This is because the loss of materia melds from syncing down higher item level gear will significantly impact damage. This is no longer an issue once gear is released of sufficiently high item level to cap both substats (around 120 item levels above the item level sync: one expansion's worth of patches).
Syncing down gear with more than two substats (e.g. Relic Weapons) works similarly in terms of the stat caps. Because of this, it is generally preferable to use a relic weapon for ultimate raids, including those released in the current expansion, as long as the weapon's item level exceeds the item level sync of the raid.
Images
-
Here the average IL can be seen next to asterisks * on the character profile page -
This Hands gear requires level 15, but has Item Level 17
References
- ↑ https://kupobot.com/faq/ ilvlsync