A character has access to one Source at character creation, and thereafter is considered a [Source] character. So if a character picks the Arcane Source at character generation, they gain the Arcane trait, and ever after are considered an Arcane character, even if they choose a second or third source later in their career.
If you want to access another Source, they must pay the Source header cost in CP before you can access that Source OR the Headers under that Source. When a character has accumulated 20 CP, you can unlock a second Source. At 80 CP, a third Source can be unlocked. A character can never unlock all 4 sources.
Again, for clarity—IGNORE the Character Point cost after the Source Header unless you are going for your second or third Source.
Example: Brekk picks the Primal source at character creation is thereafter a Primal character, regardless of what other Source he may purchase access to later. He decides after a year of adventuring (still in 1st Tier) that he wants more access to heavy armor and combat abilities, so he pays the 5 CP cost and picks up the Skill Source header. He now has access to all the skills in the Skill Source section that he qualifies for, and he can choose additional Skill-based Classes.
When he acquires 80 CP, he decides that he wants to cast arcane spells as well,so he pays the 5 CP cost again, and now he can spend the CP to buy Arcane Source skills and he can unlock any Arcane class he might want. Brekk now has the maximum number of Sources he can get, and so will never have access to the Otherworld Source.