Commander units popped in the capital of a royal side are Royals. Commander units popped by a royal side in a city other than the capital are Nobles. All other units are common. Royals that are Rulers are Kings or Queens. Commoners that are Rulers are Overlords. I would guess that a Noble Ruler would be a Regent. Royals that are not Rulers are Prince and Princesses. I don't know if a Regency can pop Royals, but it seems likely to me that it could probably pop Nobles. Any Commanders, Warlords and Casters, popped by Gobwin Knob under King Saline IV were Royals or Nobles, but all other units were common. So, Stanley, a piker popped by a Royal side, was common. He was promoted to Warlord, then Chief Warlord and then Heir Designate, but he was still common. When King Saline IV croaked, Stanley became Ruler, but since he is common, he is an Overlord. Since Sizemore was popped as a Commander, he is either a Royal or Noble, depending on if he popped at Gobwin Knob or elsewhere. I suspect elsewhere, since he never refers to Saline as his father. Wanda was popped as a commander by Overlord Firebaugh, so she is common. If Don croaks before his new heir pops, Caesar would become Regent, according to my earlier speculation.
There is some speculation here, such as whether Commander units
refers solely to Warlords or Casters as well, but most of this is based on what Saladman
said was in Book 1.