Proposed Canon
It is not uncommon for an Heir to also serve as Chief Warlord, but it is not necessary for one unit to serve both functions, and both titles can be redistributed by the Ruler.
 Known Heirs
- Goodminton - Side destroyed
It may be the case that all Heirs must be Commanders; every Heir seen thus far was either popped as a Warlord-Heir, or was named Heir Designate while serving as a Warlord or Caster. (Stanley was promoted to Warlord, and then named Heir Designate.) Naming an Heir Designate may require promoting the unit to Warlord first.
It may be that Heirs, while capable of claiming sacked and razed Capitals, may not be able to claim neutral cities. No one has spoken of claiming Unaroyal and restarting that side. Of course, this might be due to other issues. Stanley has no Heirs to claim Unaroyal. And the RCC II, while they have heirs, may not feel that the limited benefits of gaining one new unmanned city outweigh the expense of seizing and fortifying it.
Since a Warlord in production may be popped as an Heir (at the cost of additional turns), and since there is a small chance of a Warlord in production instead popping as a Caster, then it is possible that a Warlord-Heir may instead pop as a Caster-Heir.
 Multiple Heirs and Line of Succession
It is unclear if Transylvito's heirs existed concurrently or consecutively. Each was Heir for a time but later croaked. Don King then named non-Royal Warlord Caesar Borgata as Heir Designate. Later, with Borgata still serving as Heir (and Chief Warlord), Transylvito began popping a new Royal Heir.
In the case of Jetstone, it is known that three Princes (Ansom, Ossomer, and Tramennis) existed at the same time. It is clear that Ansom served Jetstone as Chief Warlord, was replaced by Ossomer after his croaking, who was replaced by Tramennis after his croaking. However, the line of succession as Heir is less clear. After the capture of Ossomer by Gobwin Knob, Parson comments that they "have [Jetstone's] heir," which could refer to either Ansom or Ossamer. After Ossamer was croaked and decrypted, Slately expends a great deal of effort and Schmuckers to name Trammenis Heir Designate, which means Trammenis did not become an Heir automatically. It is unclear if Ossamer was ever Heir, and if so, whether he inherited that position because he was popped as a "backup" Heir as part of a line of succession, or if he was named Heir Designate after Ansom was croaked and decrypted.
Slately does have thoughts on Transylvito not having a royal "line of succession," but this could be referring to Transylvito having no Royal Heir at all at the time, and thus it does not rule out the possibility that a side can have only one Heir at a time. That would mean, in the event of losing an Heir, a side must then spend a great deal of production to pop a new Heir, or spend a great deal of Schmuckers to name an Heir Designate, instead of taking either (or both) of those steps beforehand. Considering that an Heir would likely only die in a time- and money-consuming conflict, and that the primary function of an Heir is to preserve the side in the event of the Ruler croaking, this would make losing an Heir extremely problematic.
If a side can have multiple Heirs concurrently, and any Heir can claim a Capital site, it's possible that sides have Heirs not only for security, but as a threat to rivals. It lets a Side credibly threaten to destroy their Capital and start a new side. However, again assuming a side can have multiple concurrent Heirs, it may be the case that only the first in the line of succession may capture a Capital site.
Normally a Ruler can disband units immediately, with just a thought. However, Heirs can apparently "make a play for the throne". This may mean that an Heir cannot be disbanded like a normal unit, or simply that they must manipulate other Sides and Natural Allies (as Stanley may have) to gain the throne.
 Banhammer and Faq
An Heir, if it escapes the destruction of its Side, becomes a Barbarian, as happened to Jillian. If it then recaptures its original Capital, the Side may be restored (as suggested by Ansom). King Banhammer popped an Heir in response to a prediction, but this would not change the Predictamancer's claim that his Kingdom will fall. With Jillian away at the time of the Fall of Faq, she was not present to commit a suicidal attack against overwhelming forces (as her demeanor would have demanded), and so she survived the event by not being present. This leaves her free to return later with adequate forces to recapture Faq and restore her Side. This may have been Banhammer's plan all along: knowing his daughter could not flee from an overwhelming force (where a philosopher Prince would have seen the wisdom of escape and so could have been kept close), she could not be kept near him in Faq. This demanded that Banhammer create friction between himself and his daughter, allowing him to send her away to do as she would willingly while ensuring her survival; whereas, had she known he was trying to protect her, she would have resisted.
Heirs may have some protection against being disbanded by the Ruler. Even though King Banhammer doesn't seem to like Jillian, he doesn't disband her. This could be due to Jillian misunderstanding his opinion of her, or perhaps, Natural Thinkamancy imparts something like parental love. It may also have been purely an expense issue, or possibly a moral one (just because he doesn't like her doesn't mean he should disband her.)
 Popping an Heir
Designating a Heir should take just as much worth or more but in actual money, instead of production time. The price to designate Prince Tramennis as Jetstone's heir during the Assault on Jetstone would be roughly 165,000 Shmuckers. (5K*28 archons=140K + 25K bonus, assuming Gobwin Knob had not lost or gained any archons in between the Battle for Gobwin Knob and the Battle of Expository Bridge.)
These may be the only means of acquiring an Heir. However, it is possible that a popped Warlord, which has a certain percentage chance of being Noble, or Royal, and/or a caster, also has a chance of being an Heir.
 Puppet States
Heirs may claim sacked cities as their own. However, heirs also have inherent loyalty to the side that popped them. Therefore, it is possible that establishing an heir in a city may allow a side to create a puppet side, similar to what Transylvito did to Faq.