D&D 5E, which Baldur's Gate 3 is based off of, has four tiers of challenge and adventures.
Tier: Levels 1-4. These are the challenges for adventurers just starting and learning what it means to be an adventurer, get a feeling for their abilities and learning what they can and cannot do. Threats in tier 1 tend to be small problems on the outskirts of a kingdom. Problems facing rural towns. Goblin raids. Gnoll attacks. Kobolds. Bandits.
Nothing too serious, but still deadly for beginners or those barely trained.
Tier 2: Levels 5-10. These adventurers are more seasoned. The fighters in this tier will gain the ability to attack multiple times in a turn, spellcasters gain access to third-level spells, and are well on their way to becoming local heroes.
Tier 2 threats and challenges are things that threaten cities and small kingdoms/city-states.
Tier 3: Levels 11-16.
The adventurers are heroes of kingdoms and countries. They are widely considered the best of the best. The threats and challenges in this tier are threats to entire continents.
Tier 4: Levels 17-20.
This is the realm of the gods. Adventurers in this tier are in battles with the divine and face threats that challenge entire planes of existence.
