From Diablo Wiki
Inherent affixes (often called "pre-set" mods) are properties guaranteed to be on legendary items that are not Legendary Affixes, but that are always found on the item.
Legendary Affixes are special properties displayed in orange text that add bonuses not otherwise obtainable. Most legendary, set, and crafted items do *not* have such special properties. Many of them do have some Inherent Affixes though, which are usually just normal random affixes that are always tied to the item in question. Some inherent affixes are special mods not found in the random affix pool, but such items are uncommon.
For instance, the [Night's Reaping]], Crafted Legendary 1H Mighty Weapon, has no legendary affix, but there are two inherent affixes
- +(981-1199)-(1175-1490) Cold Damage
- +14-18% Life
- +4 Random Affixes
This type of weapon could randomly have rolled with that sort of cold damage, but +Life% is not normally found on weapons, so that stat could never be on the item randomly.
Crowding Out Primary Affixes
Inherent Affixes aren't always a good thing, since they usually take up primary affix slots, of which there are only 4 on most items in Diablo 3 and Reaper of Souls. This is a bad thing when the inherent affix is less valuable than a random affix that might otherwise spawn. An example of that can be seen with the Sydryu Crust, a crafted legendary bow. It has no legendary affix, but has three inherent affixes:
- +(981-1199)-(1175-1490) Poison Damage
- +5-7% Increased Attack Speed
- +9-10% damage to Elites
These are all good affixes, but the problem is they take up 3 of the 4 possible primary affixes, leaving room for only one more in Loot 2.0's 4/2 affixes system. Since top quality weapons in Diablo 3 always have +mainstat and a socket, it's impossible to get both of those affixes on this bow (or the other bow recipe, Unbound Bolt, which also has three inherent affixes, none of which are Dex or socket).
Since there's only one free primary affix, this bow will always be sub-optimal upon creation, and even if the Demon Hunter gets a socket or +Dex in the 1 available primary affix slot, they'll need to enchant one of the inherent stats (IAS or +Elite damage) into either +Dex or a socket to make this weapon comparable to the best found items.