From ErfWiki

Proposed Canon

Heir Designate is a Heir that was promoted to that status as opposed to having popped as a Heir.^{Erf-b1-p079, Erf-b1-p082}

Promotion to Heir Designate is considered a rare event and is done at great expense.^Erf-b1-p079

Speculation

The only known example of a unit promoted to Heir is Stanley.