Legal jurisdiction is a complex construct in the Third Empire. In many places, the structure of jurisdiction is more implied than theorized, leaving who has command over whom a patchwork of common law and rulings where people just *know* things. For example, knights in most lands in Arunia have the authority to arrest or detain anyone below the knightly class. However, centuries of scholarship undertaken by emperors and clerics of Haeron have elaborated the system of manicipium to a much more carefully calculated degree. Indeed, it has been codified and written in the great lawbooks of the Lawkeeper's Temple.
So the first question is a simple one, namely: what is mancipium? It's a legal concept that determines who has the authority to arrest, arraign, judge, and detain whom within the Third Empire. In its most anceint and extreme manifestation, manicipium was the authority of a noble householder to dole out death to any of his dependents regardless of the law. The rule of the patrician was literally a rule over life itself.
A compilation of First Empire law known as the Codex Theolinus (supposedly commissioned by the third emperor, Theolon the Priest) set out a rudimentary attempt to curb the power of the great noble tribes under his command. By the reign of Fabarxus (to whom we owe the grand Faberlaine Wall surrounding Pillar Hill) the concept of manicipium had been fully extended to resemble the modern 10th Age notion.
Who has manicipium?
The notion of manicipium has been narrowed from its archaic form to one that is imbued in the imperial class system. Lords have direct manicipium over their demesne; all knights who serve them, all peasants who live in their realm, all merchants who's residences are found within their realm. These lords, the Magnas, Socioari, and Dynasren (Barons, Counts, and Dukes), have the authority to judge anyone who calls their demesne home as well as any peasants from any other realm.
Knights possess this self-same power; they act as direct agents of their lords. However, while a lord has manicipium over his own knights, they cannot judge the knight-agents of other lords.
Lord's knights ------ knights of other lords (outside manicipium) ------ imperial knights (outside the lord's manicipium, but under the emperor's)
...wherein manicipium flows downward from the emperor, through the lord of a demesne. If a knight commits a crime within the realm of someone other than his own lord, the offended lord must plea to the knight's lord for restitution. Thus is established the rule known as Lex Agentia, the Law of Agents, whereby an agent can only be judged by his own master. Of course, this means that the emperor's personal servants are outside everyone's manicipium save the emperor's... and the emperor has manicipium over every subject of the empire (including agents of other lords).
Are there exceptions?
Yes. Many. The most important being the exception of urban communes and the very complex exception of the Special Commune of Miles.
Urban centers, known as communes, are generally governed by a council of merchants. Manicipium here falls under the Lex Mercantis, which was developed by merchants of that ancient Republic. Namely, all people within a city, no matter their origin, are under the manicipium of the municipal law courts... unless they are the agents of a lord from outside the city. Citizens of the communes fall under the manicipium of any lord who's lands they travel through and may be judged as though they were peasantry whenever they leave the city.
The Commune Mileas
The Commune of Miles has rules much like other communes, save knights and foreign agents committing crimes within Miles are subject to the judgement of the cities' courts. Unlike other communal citizens, people born within the circuit of the walls of Miles may only be judged by the law courts of Miles proper. If they are arraigned before a baronial, comital, or ducal court and they can prove they originate from Miles, they must be tried within the city by judges from the Temple.