I tend to agree with Deathkiss and think Fredo had no official title, but was born into "mob royalty", so to speak. Just being "blood" would carry an "unbreakable" connection to the family, yet the family, knowing Fredo, knew where to draw the line as far as any of his "duties" were concerned.
And I agree with TB as far as the senate hearing chart showing Fredo as "underboss." They just assumed he held a high position in the family.
TIS