Single-class progression
Single-class characters gain power levels every two character levels, and the source says ability tiers unlock with each power-level gain.
How it works
The class passage says single-class characters gain a power level for every two character levels. It states that the next tier of abilities unlocks each time a power level is gained, contrasting this with multiclass progression every three levels. It does not provide a complete character-level table for single-class ability unlocks or describe specific single-class abilities beyond the separate Barbarian examples.
When it helps
If you want the faster power-level cadence described in the passage, single-class progression gains power levels every two character levels.
Read the supporting references
Check the edition and source version for patch-specific details. Guide methodology.