Two demons can be chosen for fusion to create a new demon, and at least one of the demons chosen for fusion must be of Common Archetype.
As demons summoned from Superior Summons are not Common Archetypes, they cannot be chosen as both parts of normal fusion.
When demons are fused, the result will be a demon with +1 of the average grade of the two demons fused.
If the race of the demon produced doesn't have an applicable grade, then it will be one grade higher than the average +1.