Evasion Base Types
Evasion is primarily gained from gear: Helmets, Body Armours, Gloves, Boots and Shields. The majority of the Dexterity-aligned base types have inherent Evasion present as the Defence stat that's tied to the item. This portion of Evasion can be scaled by local modifiers, for example 'X% increased Evasion Rating' or '+X to Evasion Rating'. These local modifiers scale the Evasion gained from the piece of gear directly and do not apply globally to your character. The Quality stat is a multiplier on the final amount of Evasion on a piece of gear.
Local vs Global Modifiers
A local modifier is one that only applies to a stat on the item it's present on, whilst a global modifier will apply to the total stat on the character. Local modifiers are found on items with inherent stats, for example Boots with inherent Evasion will be able to roll local modifiers that improve the inherent Evasion on the item. However on items where there is no inherent stat such as Jewels, or on items that can roll modifiers that do not apply to the inherent stat, such modifiers are typically global modifiers.
For example the Amulet below has a modifier that grants '25% Increased Evasion Rating' which is applied globally to the character's flat Evasion, as it cannot apply locally to any stat on the item.

The Helmet below has '+167 to Evasion Rating' in addition to '24% increased Evasion Rating'. However, as this item has inherent Evasion, these modifiers are applied locally to increase the total flat Evasion gained from the item.

Scaling Evasion
Flat Evasion gained from equipment or other sources is added together and scaled by any global Evasion Rating modifiers you have, such as 'X% increased Evasion Rating' from the passive skill tree.
Note: modifiers to Global Defences will also scale Evasion.