From the releases:
Clad in ancient suits of power armour, impressively powerful and daunting to behold, Mark III Space Marines are a living reminder of the dark days of the Horus Heresy. Their ‘Iron’ armour is a variation on the previous generation’s ‘Crusade’ pattern, with extra armour and reinforcements allowing them to absorb an unprecedented amount of enemy firepower. When encased in this armour, a Space Marine becomes an unstoppable juggernaut.
Designed by the Mechanicum to replace the worn-down power armour used during the Great Crusade, Mark IV power armour is ancient and powerful. The most common pre-Heresy armour still in wide use, with even a few Chapters still producing new suits, it is a testament to the ingenuity and longevity of the Space Marines.
Tactical Dreadnought Armour, or Terminator Armour as it is more usually known, is the most resilient form of personal protection available to the Adeptus Astartes. Originally developed during the closing years of the Great Crusade, and adapted from the heaviest of industrial gear, several types and patterns were developed concurrently. Perhaps the most advanced form is the Tartaros pattern, allowing for greater mobility than other examples of Terminator armour with no loss in durability or protection.