From ErfWiki

[edit] Proposed Canon

"Prince" and "Princess" are Royal titles for the popped "descendants" of Kings and Queens. Not all Princes and Princesses are Heirs, though all Royal Heirs are Princes or Princesses.

Known Princes and Princesses are:

• Prince Ansom of Jetstone - Former Heir; Croaked & Decrypted, now claims no royal title
• Prince Tramennis of Jetstone - Former Heir Designate (was not popped as an Heir); now King of Jetstone
• Prince Ossomer of Jetstone - Possibly Former Heir, unknown; Croaked & Decrypted, then Turned, then finally Dusted
• Jillian Zamussels of Faq - Former Popped Heir; now Queen of Faq
• Don King's son and daughter, of Transylvito - Former Popped Heirs; Croaked

[edit] Speculation

"Prince" and "Princess" could be honorifics for all Royal units popped to a side ruled by a King or Queen.

or

A random percentage of Royal units popped on a Royal side might have the "quality" (like being Heavy) of being a Prince. (Since units popped by Royal sides have a random chance of being Noble or Royal, this might be another "step" up that ladder.)