Challenge difficulty levels

1. Gameplay Achievements vs Social Achievements (Gamification Achievement Systems Tips)
2. Player Engagement vs Player Involvement (Gamification Achievement Systems Tips)
3. Reward Timing vs Reward Frequency (Gamification Achievement Systems Tips)
4. Sandbox Games vs Linear Games (Gamification And Bloom's Taxonomy Tips)
5. Game Difficulty vs Challenge Difficulty (Gamification Achievement Systems Tips)
6. Performance Metrics vs Engagement Metrics (Gamification Achievement Systems Tips)
7. Player Challenges vs Player Objectives (Gamification Player Progression Tips)
8. Player Goals vs Game Goals (Gamification Achievement Systems Tips)
9. Session Length vs Play Time (Player-Centric Gamification Tips)