Tortoise ORM: bulk_create update_fields vs on_conflict

Question

To perform upsert operations in Tortoise, we can use the bulk_create method. For this we should provide both update_fields and on_conflict params. Like this:

await Users.bulk_create(users_list, on_conflict=fields_list, update_fields=fields_list)

The official documentation lacks examples or in-depth explanations of what the difference is between these two parameters. But it forces me to set them both. Why?