When you reach level 20 a Skill points shield will appear and an add will start flashing above your avatar tab.

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When you press the Skill points shield a new window will open.

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For every level over level 20 you get 5 potential skill points per level to allocate to the 5 characteristics with the plus sign next to them.

There are 5 categories that the potential can be added to by all classes. Below is a Breakdown of what each class gets from each category.

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There are two ways that the Stats area below the Characteristics (highlighted) improve.
The first way is by the Characteristics themselves.
The second way is by Levelling up.

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I hope you all enjoy the game as much as I do (Happy Gaming)