With each purchase, you may gain an additional Caste/Aspect from your chosen Exaltation(s), gaining the Abilities or Attributes they have an affinity for - translating into a small boost in initial power and a more noticeable boost to growth speed - as well as the Anima flares and powers tied to that caste, allowing them to use said powers individually or in any combination when expending Essence.
This upgrade also has additional benefit for Terrestrials and Exigents specifically: For Terrestrials, they receive not only their Caste Abilities and Anima powers, but also additional elements to go with them, as well as a boon in the form of counting as multiple Exalts in one, allowing them to gain more power out of their Elemental and Cooperative Charms; Exigents, due to not strictly having Castes like other Exalts, may instead gain the nature and domains of an additional god of Creation to add to their capabilities, something which would normally require illicit ‘after-market’ modifications on behalf of their patron and would come with severe complications.
(Note: Only Alchemicals (and technically Lunars) have six Castes, so purchasing this upgrade the full five times is unnecessary without the appropriate Exaltation. If you have the Machine-Dragon Symbiosis addon as well, then each purchase will grant you both an Autocthonian and Creation element, and taking all five allows you to pick a custom sixth Creation element to round out the set, such as ‘Ice’, for example)