A Progression is a list of features that a character can gain access to so long as they meet the required minimum character level. Some features can also scale based on the character's level, much like the bonus component of a Metaphor. Multiple Progressions can apply based on the character's species/race or their class/role.
A character's relevant Progressions should be recorded on the Traits and Progressions section of the Character Sheet.
There are no default Progressions at this time, but will be in the future.
Progression Guide
The Game-Master should provide options to a player when Progressions are available to ease the process. Aspects could include a character's racial-traits/ancestry, their class/role or some other aspect worthy of a unlock progression via level-ups.
Progression Feature Components
Each Feature must have Minimum-Level, so that feature may be usable from that level onward. After that the Feature then must have an Effect/Function that modifies the character's stats or properties, or gives them new options. Optionally, Features can use the Character's Level as a modifier, so that it can increase as the character's level increases.
Feature Unlocking & Choices
Features can unlock in a few ways: Single, Multiple and Choice. Single unlocks only one feature for that character level. Multiple implies that two more features are unlocked after reaching that level. Choice functions like a Single unlock except there are two or more choices and the player must pick one for that level. In the case of 'Choice' style unlocks, the character may be able to acquire all the features on subsequent levels, but sometimes features are mutually-exclusive (either for balance, or they offer contradictory effects).
Concurrent Progressions (Stacking, Distributed)
A character may have more than one Progression at a time. A Progression for a character's Race, and their chosen class/role is a common combination. However, when it comes to multiple Progressions of the same type (Multi-classing), it is up to the Game-Master to decide whether they can stack or if the character must distribute their levels amongst Progressions of the same type. If they stack, then a Lv4 character could be treated as 4th level in both classes, but if they need to distribute then they may have to be treated as 2nd level in both classes (or whichever way they wish to distribute levels).
Progressions added upon a character after creation (such as one that is bestowed, or learned later) is likely to be treated as a 'distribute' Progression and as such, level-ups are required to unlock their features.
Features
Features can be all kinds of things, from offering bonuses to checks, to outright granting and improving Metaphors to characters. They can modify a character's stats (Changing Attributes, Core Values, Status Thresholds), grant resistances (Status, Damage). Features can have static bonuses/effects, or they may be able to scale upon a character's other stats, including Attributes, Core Values, and even the character's own level.
Since Features have a minimum level, stronger features generally unlock later in a character's Progression. Although sometimes a Feature will rely on a higher level because of the way its bonus scales.
"Are Progressions required?"
No, not all games require the use of Progressions in order to function, they exist to help Game-Masters and authors create alternate ways for characters to gain options and modifiers without having to create or obtain Metaphors directly. Metaphors themselves offer enough functions to a character that they can be relied upon by themselves. Game-Masters can adjust the number of Attribute points and ATOs a level grants to compensate for a lack of Progression options if needed.