For a complete extensive in-depth look into shield theory please refer to this guide by vibe checker#5502

Elemental reactions deal % HP damage to the shield/armor
The total number of reactions (in units of U) required to break this shield/armor is listed in the table.
The color represents how efficient that element is in breaking corresponding shield/armor

Attacks typically cause 1U reactions.
Some attacks cause stronger reactions.

Examples:

Amber charged shot (2U), Bunny (2U), ult (6x1U)
Beidou E (2U), ult (4U)
Kaeya E (2U)

Note that Electro-charged reaction values listed in the table are higher than other reactions.
This is due to the fact that each EC tick consumes 0.4U of the elemental gauge, which decays
naturally over a time it takes for EC ticks to occur.

Internal Cooldown (ICD) affects how often you can apply an element
and thus how fast you can break a shield/armor.

Tip: use different attacks to break shields faster.
Example: Lisa's normal attack and charged attack have different ICDs
so it's optimal to alternate between normal and charged attack

Terminology

Shield = blocks all incoming damage
Armor = reduces incoming damage
Fragile (shield/armor) = can be broken with heavy (blunt) attacks

means that this shield/armor is fragile, meaning it can be broken with heavy attacks.
Blue color represents the fact that only heavy attacks can destroy it because
heavy attacks can cause Shatter reaction.

means that this shield/armor is fragile but aside from heavy attacks it can also
be broken with light attacks. Light attacks do neglectable damage though.

Light attacks include sword basic attacks, sword charged attacks, Jean E, Anemo MC E, Bennet E etc

Heavy attacks include Geo attacks, plunge attacks, claymore basic attacks, most claymore users' abilities, Amber E,
Overload explosion. Heavy attacks are able to cause Shatter reaction.

Both light and heavy attacks are characterized by their ability to stagger enemies.
The stagger values of different attacks vary and correspond to their ability to break fragile shields/armor.

Geo Slimes, Geo Fatui and Geo Lawachurls have Geo shields/armor that interact with Crystallize
in the following way: Geo attacks cause a Crystallize reaction that consumes whatever gauge is left
on the enemy. The strength of the trigger does not matter. See example below.

Interaction between Crystallize and fragile shields/armor:
Geo attacks are both heavy and can cause Crystallize reactions.
It means that they can deal two types of damage at the same time.

Example: Kaeya uses his E on shielded Geo Slime, applying 2U Cryo aura.
Then while you switch characters and perform another attack, the gauge naturally decays to ~1.4 (for example).
Then you perform a Geo attack causing a Crystallize reaction, dealing bonus damage = 35% of Max HP (1.4U / 4U)

plus another instance of damage due to Geo being a heavy attack and Geo slime's shield being fragile.

Domains (including Abyss): certain enemies in certain domains have higher shield/armor durability.
Examples: Cryo Abyss mages in 2nd wave of Cryo domain, in 9-1-2 and 10-1-1 (in patch 1.5 Abyss)
Co-op Domains: enemies in co-op domains typically have higher shields/armor durabilty

Knowns bugs/interactions:
1) Swirled elements typically have high U values (Anemo MC can destroy a Cryo Abyss mage shield in a
single use of E by swirling Pyro onto the shield from a nearby enemy).

2) Wet Cryo shields/armor take zero damage from Crysallize and Swirl and ~half damage from Melt
3) Freezing enemies with Hydro shield/armor makes them invincible to Electro-charged due to Frozen
status (applying Electro causes Superconduct that deals no damage to the underlying shield/armor)